Compression of video signals

ABSTRACT

A digital video signal is compressed by spatial sub-band filtering to form data sets constituting respective sub-bands of the two-dimensional spatial frequency domain. The data sets for a field or frame are stored. A first sequencer controls writing, in accordance with a desired sequence, of the stored data to a quantizer in which they are quantized in accordance with respective values, those values being such that the amount of quantization of at least a data set constituting a sub-band to which dc luminance information of the signal is at least predominantly confined is less than the average of the amounts of quantization of the remaining data sets. The quantized data sets are then encoded in an entropy encoder which has a first coding portion for coding quantized data representative of dc luminance information and a second coding portion for coding quantized data representative of ac luminance information. A second sequencer, which may be the same sequencer as the first sequencer, controls operation of the quantizer so that each datum (sample) written thereto is appropriately quantized, and controls operation of the entropy encoder so that each quantized sample is directed to the appropriate one of the first and second coding portions.

CROSS-REFERENCE TO RELATED APPLICATION

Reference is made to copending U.S. patent application Ser. No.07/810,337, which corresponds to UK Patent Application No. 9100591.8filed Jan. 11, 1991 and is assigned to the assignees hereof, which wasfiled on the same day as the present application, and which includesclaims directed to the following disclosure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the compression of video signals.

2. Description of the Prior Art

Compression of video signals on an intra-image basis (for example,compression on an intra-field or intra-frame basis) makes use of theredundancy present in pictures or images represented by the signals toreduce the amount of information needed to represent the pictures orimages. The compression can be used to reduce bandwidth, in the case oftransmission of a video signal, or to reduce storage capacity, in thecase of storage of a video signal.

Intra-image compression can, as is known, be effected in the time domainby the use of differential pulse code modulation, in which a predictoris used to predict the values of samples representing pixels based onprevious pixel values. Since the image pixels are highly correlated, theprediction is accurate and results in a small and uncorrelated error(that is, a difference between the predicted and actual values). Theerror samples are encoded and, since they can be encoded using fewerbits than the samples representing the original pixels, compression canbe achieved.

FIG. 1 of the accompanying drawings shows a known apparatus or systemfor effecting intra-image compression of a video signal in thetwo-dimensional spatial frequency domain. A video signal, which is indigital form and comprises successive multi-bit (for example 8-bit)samples or words each representing a respective pixel of an scannedimage or picture, is applied via an input 10 to a decorrelator 12. Adecorrelated version of the video signal is outputted by thedecorrelator 12 to a quantizer 14 and then to an entropy encoder 16,which together compress the decorrelated version of the video signaloutputted by the decorrelator 12 to produce a compressed signal of anoutput 18. The compressed signal can then be transmitted or stored.(Note that, although the decorrelator 12, quantizer 14 and entropyencoder 16 are shown for clarity as being separate items, they may inpractice be embodied in an at least partially combined form.) Aftertransmission or storage, the compressed signal can be restoredsubstantially to its original form by expansion by way of entropydecoding, dequantizing and correlation operations which employparameters converse to those used for decorrelation, quantization andentropy encoding, respectively, upon compression.

The operation of decorrelation performed in the decorrelator 12 reliesupon the fact that neighboring pixels of an image are highly correlated,whereby processing an image (for example, a field or frame of a videosignal) to form decorrelated signal portions representing differentcomponents of the image in the two-dimensional spatial frequency domainenables a reduction in the amount of information needed to represent theimage. Specifically, the decorrelated signal portions representdifferent spatial frequency components of the image to which the humanpsychovisual system has respective different sensitivities. Thedifferent decorrelated signal portions are subjected to differentdegrees of quantization in the quantizer 14, the degree of quantizationfor each signal portion depending upon the sensitivity of the humanpsychovisual system to the information in that portion. That is, each ofthe decorrelated signals is quantized in accordance with its relativeimportance to the human psychovisual system. This selective quantizationoperation, which is a lossy operation in that it involves deliberatediscarding of some frequency data considered to be redundant or oflittle importance to adequate perception of the image by the humanpsychovisual system, in itself enables some signal compression to beachieved. The quantizer 14 enables compression to be achieved in twoways: it reduces the number of levels to which the data inputted to itcan be assigned, and it increases the probability of runs of zero valuesamples on the data it outputs. Note that, in video signal compressionapparatus described in detail below, the ability to achieve signalcompression provided by the operation of the quantizer 14 is not used toproduce a bit (data) rate reduction in the quantizer itself. Instead, inthat case, the ability to achieve signal compression provided by theoperation of the quantizer is carrier into effect in the entropy encoder16 in that the reduction in information content achieved in thequantizer 14 enables a consequential bit (data) rate reduction to beachieved in the entropy encoder.

Further (non-lossy) compression, and bit (data) rate reduction, isprovided in the entropy encoder 16 in which, in known manner, using forexample variable length coding, the data produced by the quantizer 14 isencoded in such a manner that more probable (more frequently occurring)items of data produce shorter output bit sequences than less probable(less frequently occurring) ones. In this regard, the decorrelationoperation has the effect of changing the probability distribution of theoccurrence of any particular signal level, which is substantially thesame as between the different possible levels before decorrelation, intoa form in which in which it is much more probable that certain levelswill occur than others.

The compression/coding system or apparatus as shown in FIG. 1 can beembodied in a variety of ways, using different forms of decorrelation.An increasingly popular form of implementation makes use of so-calledtransform coding, and in particular the form of transform known as thediscrete cosine transform (DCT). (The use of DCT for decorrelation is infact prescribed in a version of the compression system of FIG. 1described in a proposed standard prepared by JPEG (Joint PhotographicExperts Group) and currently under review by the ISO (InternationalStandards Organization).) According to the transform technique ofdecorrelation, the signal is subjected to a linear transform(decorrelation) operation prior to quantization and encoding. Adisadvantage of the transform technique is that, although the wholeimage (for example, a whole field) should be transformed, this isimpractical in view of the amount of data involved. The image (field)thus has to be divided into blocks (for example, of 8×8 samplesrepresenting respective pixels), each of which is transformed. That is,transform coding is complex and can be used on a block-by-block basisonly.

A recently proposed approach to compression/coding in the frequencydomain is that of sub-band coding. In this approach, the decorrelator 12in the system of FIG. 1 would comprise a spatial (two-dimensional)sub-band filtering arrangement (described in fuller detail below) whichdivides the input video signal into a plurality of uncorrelatedsub-bands each containing the spatial frequency content of the image ina respective one of a plurality of areas of a two-dimensional frequencyplane of the image, the sub-bands then being selectively quantized bythe quantizer 14 in accordance with their positions in the sensitivityspectrum of the human psychovisual system. That is, decorrelation isachieved in this case by putting the energy of the overall image intodifferent sub-bands of the two-dimensional spatial frequency domain.Sub-band filtering is believed to provide better decorrelation than thetransform approach. Also, unlike the transform technique, there is norestriction to operation on a block-by-block basis: the sub-bandfiltering can be applied directly to the video signal.

The proposed JPEG standard mentioned above requires that datadecorrelated by means of a block-by-block DCT transformation operationbe quantized and then entropy encoded. The data is required by thestandard to be quantized and encoded in a particular sequence dictatedby the order in which it is outputted by the decorrelator, which orderis in turn dictated by the way in which the input digital video signalis divided into blocks for transformation. The sequence is such thateach successive one of a sequence of (for example) 8×8 arrays (blocks)of data each resulting from transformation of an 8×8 block of data ofthe input signal is, after quantization, entropy encoded in such amanner that one of the 64 data in the array is entropy encoded in amanner different than the other 63 data in the array. This requiresswitching of the entropy encoding at the block frequency, that is onceevery 64 data (samples). Moreover, the sequence is such that the 64 datain each block (in turn) are quantized and encoded in a particular orderspecified in the standard. In the case of sub-band filtering, the datato be quantized and encoded is of a very different format than thatobtained in the case of block-by block DCT transformation. On the faceof it, the JPEG standard approach not only excludes the use of sub-bandfiltering, but, on the face of it, is wholly incompatible with the useof sub-band filtering, which does not require a block-by-block approach.This is unfortunate because, as mentioned above, sub-band filtering isbelieved superior to block-by block DCT transformation.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the invention is to provide a technique for compressing avideo signal which is of a flexible nature.

Another object of the invention is to provide a technique forcompressing a video signal which enables the use of sub-band filtering(as opposed to block-by-block orthogonal transformation) fordecorrelation, and yet also allows the compression to follow (ifdesired) the general format prescribed by the above-mentioned JPEGstandard.

The invention provides a method of compressing a video signal, in whicha digital video signal is subjected to spatial two-dimensional sub-bandfiltering to form a plurality of data sets constituting respectivesub-bands of the two-dimensional spatial frequency domain. These datasets for a field or frame of the video signal are stored, and the storeddata sets are quantized in accordance with respective values, thesevalues being such that the amount of quantization of one of the datasets constituting a sub-band to which dc luminance information of thesignal is at least predominantly confined is less than the average ofthe amounts of quantization of the remaining data sets. The stored dataare written to a quantizer, for carrying out the quantizing, in adesired sequence, and the quantizing is controlled in accordance withthat desired sequence such that each datum written to the quantizer isappropriately quantized. The quantized data sets are entropy encodedsuch that quantized data representative of dc luminance information iscoded by a first coding technique and quantized data representative ofac luminance information is coded by a second coding technique. Theentropy encoding is controlled in accordance with the above-mentioneddesired sequence such that each quantized datum is subjected to theappropriate one of the first and second coding techniques.

The flexibility resulting from the fact that the data sets for a fieldor frame are stored and then written to the quantizer in a desiredsequence, and a realization that, while their respective formats arevery different, the data obtained in the respective cases of sub-bandfiltering and block-by block DCT transformation have sufficientresemblance (at least in terms of information content) to one anotherthat the former can be rearranged to resemble the latter, to a greateror lesser degree, means that the apparent incompatibility betweensub-band filtering and the JPEG standard is not as great as at firstappears. On the contrary, the same general approach as stipulated in theJPEG standard can, to a greater or lesser degree, be adopted.

The extent to which the JPEG standard is followed depends upon theapplication. If, for example, strict compliance with the standard is notrequired, for example if a proprietary piece of recording equipment isto be produced in which the designer can specify at his discretion theway in which the signal is to be compressed and subsequently expanded,there is no need to follow the above-described sequence of dataquantization and entropy encoding laid down in the standard. Thedesigner can use a sequence that seems best in the circumstances.

Thus, according to one form of implementation of the invention describedhereinbelow:

the desired sequence in which the stored data is written to thequantizer is such that

(i) all the data of said one of the stored data sets constituting thesub-band to which the dc luminance information is at least predominantlyconfined are quantized,

(ii) after which, upon each occurrence of an operation carried out for anumber of times equal to the number of data in each stored data set,those data corresponding to a respective one of the spatial positions ineach of the remaining stored data sets are quantized in a predeterminedorder;

the quantizing is controlled such that all the data written to thequantizer in step (i) are quantized in the same amount, namely in theamount appropriate to the sub-band to which the dc luminance informationis at least predominantly confined, and each of the data written to thequantizer in each of the operations of step (ii) is quantized in anamount appropriate to the data set of which it forms a part; and

the entropy encoding is controlled such that all the data quantized uponbeing written to the quantizer in step (i) are subjected to the firstcoding technique and all the data quantized upon being written to thequantizer in all the operations of step (ii) are subjected to the secondcoding technique.

The use of the two steps or stages set out at (i) and (ii) forquantizing the data means that the format of the data to be quantized isvery different than in the case of the JPEG (DCT) standard; and that theentropy encoding has to be switched at the field or frame frequencyrather than at the much higher frequency (block frequency) used in thecase of the JPEG (DCT) standard. However, this form of sequencing isbelieved superior to the JPEG sequence at least in some cases, in thatit groups the dc and ac information together rather than interminglesit; and the invention enables this form of sequencing to be used ifdesired.

Even if the above form of sequencing is used, the ac data can be treatedin a generally similar manner to that specified in the JPEG standard.Thus, for example, the aforesaid predetermined order may comprisesuccessive groups of the remaining stored data sets in a sequence, asbetween the groups, of sub-bands containing ac luminance information ofincreasing spatial frequency. In fact, to accomplish a form of zig-zagsequencing generally similar to that specified in the standard, saidgroups may comprise legs of a zig-zag pattern connecting the data of thesame spatial position in the different stored data sets.

According to another form of implementation of the invention describedhereinbelow:

the aforesaid desired sequence in which the stored data is written tothe quantizer is such that, upon each occurrence of an operation carriedout for a number of times equal to the number of data in each storeddata set, those data corresponding to a respective one of the spatialpositions in each of the stored data sets are quantized in apredetermined order;

the quantizing is controlled such that each of the data written to thequantizer in each of said operations is quantized in an amountappropriate to the data set of which it forms a part; and

the entropy encoding is controlled such that, for each of saidoperations, that one of the quantized data forming part of the data setconstituting the sub-band to which the dc luminance information is atleast predominantly confined is subjected to the first coding techniqueand all of the other data are subjected to the second coding technique.

In this case, the sequencing is very similar to that specified in theJPEG standard, in particular if the aforesaid predetermined ordercomprises successive groups of the stored data sets in a sequence, asbetween the groups, of sub-bands containing ac luminance information ofincreasing spatial frequency; and if said groups comprise legs of azig-zag pattern connecting the data of the same spatial position in thedifferent stored data sets.

Thus, as explained in more detail below, the format of the data to bequantized is very similar to that in the case of the JPEG (DCT)standard, which has the advantage that quantization can be sequenced ina very similar or even identical way to that used in the case of thestandard. Thus, it may be possible to use an "off the shelf" quantizerchip or assembly intended for use in a JPEG compression apparatus(possibly with changes in the quantization values). Also, in this case,the entropy encoding has to be switched at the frequency of carrying outthe operations in which those data corresponding to a respective one ofthe spatial positions in each of the stored data sets are quantized,that is at a frequency determined by the number of data sets(sub-bands), rather than at the field or frame frequency. If, as in thecase of embodiments of the invention described below, the number of datasets (sub-bands) is the same (8×8=64) as the number of samples per blockas specified in the JPEG standard, the frequency of switching theentropy encoding is the same as the frequency (block frequency) used inthe case of the JPEG (DCT) standard. Thus, it may be possible to use an"off the shelf" entropy encoder chip or assembly intended for use in aJPEG compression apparatus.

As is well known, a color video signal can be in component or compositeform. A component color video signal comprises three separate signalwhich together represent the totality of the video information. Thethree separate signals may, for example, be a luminance signal and twocolor difference signals (Y, Cr, Cb) or three signals each representinga respective color (R, G, B). A composite color video signal, on theother hand, is a single signal comprising all the luminance andchrominance (color) information.

Previously proposed color video signal compression systems as describedabove all operate on component signals only. That is, taking the exampleof the system of FIG. 1, three separate systems as shown in FIG. 1 areneeded, one for each of the three components. Also, if the signal is incomposite form, there is a need for means to convert it into componentform prior to compression. Further, three expansion systems are neededto convert the transmitted or stored signals back to their originalform, together with (if appropriate) means to convert the componentsignals back into composite form. The need to process the video signalin component form thus involves the expense and inconvenience ofconsiderable hardware replication.

While the invention is applicable in the case of component (ormonochrome) video signals, a preferred feature of the invention is thatit can be used also to compress composite color video signals. Thispreferred feature takes advantage of a realization by the inventorsthat, due to the way in which luminance and chrominance information arecombined in conventional broadcast standard (for example, NTSC and PAL)composite color video signals, such a signal can be spatially sub-bandfiltered such that the chrominance information can be (as is explainedin detail below) concentrated in a certain area of the two-dimensionalspatial frequency domain (that is, in certain of the sub-bands), wherebyif the data sets to which the dc chrominance information and dcluminance information are at least predominantly confined are quantizedmore lightly than the other data sets (which contain wholly or largelyonly the ac luminance information) are on average quantized, then sincethe dc information is more important to satisfactory appreciation of theimage by the human pyschovisual system than the ac luminance informationit is in fact (surprisingly) possible satisfactorily to compress acomposite color video signal directly, that is without first convertingit to component form and compressing each component individually.

Another advantage of the sub-band approach to signal decorrelation isthat (as is also the case for the DCT approach) the sub-band approach isseparable between two orthogonal spatial directions. Thus, the digitalvideo signal is preferably separately spatially sub-band filtered inrespective orthogonal spatial directions. The separable approachsimplifies design.

A further advantage of the separable approach is that it enables themethod to be performed such that a field or frame of the digital videosignal is sub-band filtered in one of the orthogonal directions in afirst one-dimensional sub-band filter arrangement, stored, and thentransposed and sub-band filtered in the other of the orthogonaldirections in a second one-dimensional sub-band filter arrangement whichis of substantially the same construction as the first one-dimensionalsub-band filter arrangement.

To further minimize hardware requirements, according to a preferred formof the method, in a first stage of the filtering in each of theorthogonal directions, the digital video signal is subjected to low passfiltering followed by decimation by two and also to high pass filteringfollowed by decimation by two, thereby to produce two intermediateoutputs, and, and in at least one subsequent stage of the filtering ineach of the orthogonal directions, each of the intermediate outputsproduced in the previous stage is subjected to low pass filteringfollowed by decimation by two and also to a high pass filtering followedby decimation by two. The use of such a "tree" or "hierarchial"structure of so-called quadrature mirror filters (QMFs), as opposed tothe alternative possibility of a band of filters operating in parallel,is preferred in that it reduces hardware requirements and also enablesbetter reconstruction of the signal to be achieved on subsequentexpansion. In this regard, the aliasing that will of necessity beintroduced in the hierarchial QMF filtering or decomposition during thecourse of compression can in principle be removed completely during thecourse of a converse composition operation performed upon expansion(after transmission or storage of the compressed signal).

According to an alternative to the storage and transposition approach,the digital video signal may be sub-band filtered in a firstone-dimensional sub-band filter arrangement configured to sub-bandfilter the signal in one of the orthogonal directions, and then sub-bandfiltered in a second one-dimensional sub-band filter arrangementconfigured to sub-band filter the signal in the other of the orthogonaldirections. This approach may be preferable if the filter structure isconstructed as a unit on silicon rather than by wiring together separateintegrated circuits.

The location in the two-dimensional spatial frequency domain of the dcchrominance information is determined by the relationship between thefrequency at which an analog composite color video signal has beensampled to form the digital composite color video signal, and thefrequency of a color sub-carrier frequency of the composite color videosignal. According to a preferred form of the method, the digitalcomposite color video signal has been formed by sampling an analogcomposite color video signal at a frequency equal to four times thefrequency of a color sub-carrier frequency of the composite color videosignal.

According to the preferred form of the method disclosed below, theplurality of sub-bands make up a square array in the two-dimensionalspatial frequency domain. The array may, for example, be a 4×4 array oran 8×8 array. However, it is equally feasible, and may in some cases beappropriate, to have different numbers of sub-bands in the twoorthogonal directions, that is to employ a non-square, rectangulararray. For instance, the sub-bands may make up, in the two-dimensionalspatial frequency domain, a rectangular array having a dimension of 8 inthe direction of scanning of the video signal and a dimension of 4 inthe direction orthogonal thereto.

The invention also provides apparatus for compressing a video signal.The apparatus comprises a spatial two-dimensional sub-band filteringarrangement that filters a digital video signal to form a plurality ofdata sets constituting respective sub-bands of the two-dimensionalspatial frequency domain, a store for storing the data sets for a fieldor frame of the video signal, a quantizer that quantizes the stores datasets in accordance with respective values, those values being such thatthe amount of quantization of one of the data sets constituting asub-band to which dc luminance information of the signal is at leastpredominantly confined is less than the average of the amounts ofquantization of the remaining data sets, an entropy encoder that encodesthe quantized data sets, the entropy encoder comprising a first codingportion for coding quantized data representative of dc luminanceinformation and a second coding portion for coding quantized datarepresentative of ac luminance information, and sequencing means thatcontrols writing of the stored data from the store to the quantizer in adesired sequence, controls operation of the quantizer in accordance withthat desired sequence such that each datum written thereto isappropriately quantized, and controls operation of the entropy encoderin accordance with the aforesaid desired sequence such that eachquantized datum is directed to the appropriate one of the first andsecond coding portions.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the inventionwill be apparent from the following detailed description of illustrativeembodiments thereof, which is to be read in conjunction with theaccompanying drawings, in which like references indicate like itemsthroughout, and in which:

FIG. 1 shows a video signal compression apparatus or system forachieving intra-image compression of a video signal in the frequencydomain;

FIG. 2 is a block diagram of one form of implementation of adecorrelator, in the form of a sub-band filtering arrangement, for usein the video signal compression apparatus;

FIG. 3 is a detailed block diagram of a horizontal filter arrangementforming part of the sub-band filtering arrangement shown in FIG. 1;

FIG. 4 shows a sub-band filtered field of a video signal (luminanceonly) on a two-dimensional frequency plane;

FIG. 5 is a block diagram of another form of implementation of adecorrelator, in the form of a sub-band filtering arrangement, for usein the video signal compression apparatus;

FIG. 6 is a graph representing the response of the human psychovisualsystem to different spatial frequencies;

FIG. 7 represents a quantization matrix that would be used in aquantizer of the video signal compression apparatus if a sub-bandfiltered component (luminance) video signal were being processed in thequantizer, and shows also respective modifications to be made if,instead, a sub-band filtered composite video signal (NTSC or PAL) werebeing processed in the quantizer;

FIG. 8 is a block diagram of the quantizer;

FIG. 9 shows part of FIG. 4 on an enlarged scale, and is used to explainthe operation of the quantizer;

FIG. 10 is a diagram showing how zig-zag scanning of the ac sub-bands iscarried out in the quantizer;

FIG. 11 shows the format of quantized data emerging from the quantizerfor ac sub-bands;

FIG. 12 is a block diagram of an entropy encoder forming part of thevideo signal compression apparatus;

FIG. 13 is a representation of the contents of a fixed length codelook-up table forming part of the entropy encoder;

FIG. 14 shows a sub-band filtered field of an NTSC composite color videosignal, sampled at four times its color sub-carrier frequency, on thetwo-dimensional frequency plane;

FIG. 15 is a graph showing the two-dimensional frequency content of afield of an analog NTSC composite color video signal;

FIG. 16 shows a frame of an NTSC composite color video signal, sampledat four times the color sub-carrier frequency, on the two-dimensionalfrequency plane;

FIG. 17 is a view corresponding to FIG. 4, but showing on thetwo-dimensional frequency plane both the sub-band filtered field of anNTSC composite color video signal, and a sub-band filtered field of aPAL composite color video signal, each sampled at four times its colorsub-carrier frequency;

FIG. 18 shows how samples making up a field or frame of a digital videosignal are divided into blocks to be processed by a linear transform,for example a DCT;

FIG. 19 is a block diagram of a linear transform decorrelator;

FIG. 20 is a block diagram of a linear transform decorrelator, like thatof FIG. 19 but modified to emulate decorrelation performed by sub-bandfiltering;

FIG. 21 shows an 8×8 block of coefficients (samples) outputted by alinear transform circuit of the decorrelator of FIG. 20; and

FIG. 22 shows how 8×8 blocks of coefficients outputted by the lineartransform circuit are written into a store of the decorrelator of FIG.20.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A method and apparatus for compressing a digital video signal will nowbe described with reference to the drawings. The basic construction ofthe apparatus is in accordance with FIG. 1 (described above). Thedecorrelator 12 of the present apparatus is constituted by a sub-bandfiltering arrangement which, according to one form of implementation asshown in outline form at 12A in FIG. 2, comprises a horizontal filterarrangement 20A, an intermediate field store 22, a transpose sequencer(address generator) 24, a vertical filter arrangement 26A, an outputfield store (FS) 28 and an output sequencer (address generator) 29A. Asexplained above, sub-band filtering can be effected on a separablebasis. Thus, in FIG. 2, filtering in the two orthogonal imagedirections, namely the horizontal direction (the direction of imagescanning in the case of conventional video) and the vertical direction,is effected entirely independently and separately of one another byrespective one-dimensional filtering operations performed in thehorizontal and vertical filter arrangements 20A and 26A, respectively.

The horizontal filter arrangement 20A and vertical filter arrangement26A can be of substantially the same construction as one another. Thus,the construction of the horizontal filter arrangement 20A only will bedescribed in detail.

It will be assumed that the filtering is to achieve 8 sub-bands in eachof the horizontal and vertical directions, that is to say that a squarearray of 64 (8×8) sub-bands is to be produced. It will further beassumed that the 64 sub-bands are (as is preferred) to be of equalextent to one another.

The horizontal filter arrangement 20A is preferably of a tree orhierarchical structure as shown in FIG. 3, comprising three successivefilter stages 30, 32 and 34.

The first stage 30 comprises a low pass filter (LPF) 36 and a high passfilter (HPF) 38, each of which is followed by a respective decimator(DEC) 40. The LPF filter 36, HPF filter 38 and the decimators 40together make up a quadrature mirror filter (QMF). Each of the filters36 and 38 can be a finite impulse response (FIR) filter of conventionalform. In use, a line of a field of the input digital video signal isapplied, sample-by-sample, to the first stage 30, to be low passfiltered and high pass filtered by the LPF 36 and HPF 38, respectively.Thus, the LPF 36 and HPF 38 produce outputs comprising low pass filteredand high pass filtered versions of the input line, respectively, theoutputs representing the spatial frequency content of the line in theupper and lower halves of the horizontal spatial frequency range. Thatis, the first stage 30 divides the input line into two sub-bands in thehorizontal direction. The decimators 40 decimate (sub-sample) therespective outputs by a factor of two, whereby the total number ofsamples outputted by the decimators 40 (together) is the same as thetotal number of samples in the line.

The second stage 32 is of similar construction to the first stage 30,except that there are two QMFs each as in the first stage and the outputfrom each of the decimators 40 of the first stage is passed as an inputto a respective one of the two QMFs. Thus, the second stage 32 producesfour outputs representing the spatial frequency content of the line infour equal quarters of the horizontal spatial frequency range. That is,the second stage 32 further divides the two sub-bands, into which theinput line was divided in the first stage 30, into four sub-bands in thehorizontal direction. The four decimators of the second stage 32decimate (sub-sample) the respective outputs by a factor of two, wherebythe total number of samples outputted by the decimators of the secondstage (together) is the same as the total number of samples in the line.

The third stage 34 is of similar construction to the first stage 30,except that there are four QMFs each as in the first stage and theoutput from each of the four decimators of the second stage 32 is passedas an input to a respective one of the four QMFs. Thus, the third stage34 produces eight outputs representing the spatial frequency content ofthe line in eight equal one-eighths of the horizontal spatial frequencyrange. That is, the third stage 34 divides the four sub-bands into whichthe input line was previously divided into the required eight sub-bandsin the horizontal direction. The eight decimators of the third stage 34decimate (sub-sample) the respective outputs by a factor of two, wherebythe total number of samples outputted by the decimators of the thirdstage (together) is the same as the total number of samples in the line.

The eight outputs of the third stage 34, that is of the horizontalfilter arrangement 20A, are passed to a intermediate field store 22 andstored at positions corresponding to respective one-eighths of a firstline thereof. The above process of horizontal filtering is then repeatedfor all the other lines of the field of the input digital video signal.This results in the intermediate field store 22 containing a version ofthe field of in the input digital video signal that has been filteredinto eight sub-bands in the horizontal direction (only). Each line ofthe field stored in the intermediate field store 22 is divided intoeight portions each containing the horizontal spatial frequencyinformation in a respective one of eight sub-bands of the horizontalspatial frequency range of the image that the original fieldrepresented. Thus, the horizontally filtered field stored in theintermediate field store 22 can be considered to be divided into eightcolumns.

Referring back to FIG. 2, the horizontally filtered field stored in theintermediate field store 22 is then fed (under the control of thetranspose sequencer 24) into the vertical filter arrangement 26A, inwhich it is filtered into eight sub-bands in the vertical direction insimilar manner to that in which filtering into eight sub-bands in thehorizontal direction was achieved in the horizontal filter arrangement20A. The horizontally and vertically filtered field is fed on aline-by-line basis into the output field store 28 to be passed fromthere to the quantizer 14A. The store 28 can be considered to have beenpartitioned into an array of 64 (8×8) storage regions, in each of whicha respective on of the 64 sub-bands is stored. Thus, successive fieldsof in the input digital video signal are sub-band filtered and passed,duly filtered, to the quantizer 14 after a delay of two field intervals.

The transpose sequencer 24 produces read addresses for the intermediatefield store 22, to control reading of the contents thereof into thevertical filter arrangement 26A, as follows. As will be recalled, thesignal as stored in the intermediate field store 22 comprises the linesof the original field, each divided horizontally into eight sub-bands.That is, the signal as stored in the intermediate field store 22 can, asmentioned above, be considered to comprise eight columns. To enable thesignal stored in the intermediate field store 22 to be verticallyfiltered by hardware of the same construction (the vertical filterarrangement 26A) used to horizontally filter it, it must be transposed,that is rotated through 90 degrees, as it is read to the vertical filterarrangement 26A, so that it comprises eight rows (as opposed tocolumns). The transpose sequencer 24 addresses the intermediate fieldstore 22 in such a manner as to accomplish this.

The horizontally and vertically filtered field stored in the outputfield store 28, which has been sub-band filtered by a factor of eight inboth directions, can thus be considered as having been divided intoeight rows and eight columns, that is into an 8×8 sub-band array. Thehorizontally and vertically sub-band filtered field, as stored in theoutput field store 28 of the sub-band filtering arrangement 12 ready forquantization, can be represented (subject to the qualification mentionedbelow concerning sub-band scrambling) on a two-dimensional frequencyplane as shown in FIG. 4. In conventional manner for considering image(two-dimensional) signals, frequency is represented in normalised formin FIG. 4, the symbol pi being equivalent to half the Nyquist limitsampling frequency. For the time being, it is assumed that the inputdigital video signal is a component (luminance) signal, or even amonochrome signal, rather than a composite signal. Thus, the 64sub-bands comprise a single sub-band, referred to hereinafter as the dc(zero spatial frequency) sub-band, which contains most or all of the dcinformation image intensity data, namely the sub-band (shown shaded) inthe upper left hand corner of FIG. 4, together with 63 ac sub-bandswhich contain edge data, that is components of the two-dimensionalfrequency spectrum of the image in respective sub-bands higher than dc(zero spatial frequency). In this regard, if the filtered signal in theoutput field store 28 were viewed on a monitor, it would beintelligible. Thus, a very heavily filtered version of the originalsignal would be seen in the upper left hand corner picture area (dcsub-band) and higher frequency components could be observed in the other63 picture areas (ac sub-bands).

The sub-band filtering arrangement structure described above withreference to FIG. 3 (unlike an alternative arrangement described belowwith reference to FIG. 5), because of its hierarchical QMF structure,"scrambles" the order or sequence of the sub-bands. That is, due to afrequency inversion that takes place in each of the QMFs, if a field ofthe filtered signal in the output field store 28 were viewed on amonitor, there would not be a one-to-one correspondence between thefield as viewed and the showing of FIG. 4. Thus, while the dc sub-bandwould remain in the upper left-hand corner, the frequency planelocations of the 63 ac sub-bands would be different from (that is,scrambled with respect to) their locations in FIG. 4. The locationswould of course by the same for successive fields and can readily bedetermined from the structure of FIG. 3. In other words, while each ofthe 64 storage regions into which the store 28 is partitioned stores arespective one of the 64 sub-bands, the relative positioning of the 63storage regions containing the ac sub-bands is scrambled (in a knownmanner) with respect to the relative positioning of the ac sub-bands asshown in FIG. 4.

In order that the scrambled locations of the 63 ac sub-bands aredescrambled (that is, put into the pattern shown in FIG. 4) before thesub-band filtered signal is passed to the quantizer 14A, the outputssequencer 29A (which can be located, as shown, in the sub-band filteringarrangement 12A, though it could be located elsewhere, for example inthe quantizer 14), which is connected to the output field store 28 toproduce read addresses therefor to cause the data therein to be read outto the quantizer 14A, is so designed that the data is read out in adescrambled manner, that is in such a manner that the sub-bands assupplied to the quantizer conform to FIG. 4. (The operation of thesequencer 29A in this regard is described in more detail below withreference to FIGS. 9 and 10.)

FIG. 5 shows at 12B a form of implementation of the sub-band filteringarrangement which can be used instead of that (12A) described above withreference to FIGS. 2 and 3. The sub-band filtering arrangement 12Bcomprises a horizontal filter arrangement 20B, a vertical filterarrangement 26B, and output field store 28, and an output sequencer 29B.As in the case of the sub-band filtering arrangement 12A of FIGS. 2 and3, filtering in the horizontal and vertical directions is in this casealso effected entirely separately of one another, namely by respectiveone-dimensional filtering operations performed in the horizontal andvertical filter arrangements 20B and 26B, respectively.

The horizontal filter arrangement 20B is of a conventional FIRstructure, comprising a chain of an appropriate number of one-sampledelay elements 40a, 40b, . . . 40n tapped off to multipliers 42a, 42b, .. . 42n+1 (supplied with respective appropriate weighting coefficientsWC) whose output signals are summed by adders 44a, 44b, . . . 44n toproduce a horizontally sub-band filtered output signal 45; at the outputof the final adder. Similarly, the vertical filter arrangement 26B is ofa conventional FIR structure, comprising a chain of an appropriatenumber of one-line delay elements 46a, 46b, . . . 46m tapped off tomultipliers 47a, 47b, . . . 47m+1 (supplied with respective appropriateweighting coefficients WC) whose output signals are summed by adders49a, 49b, . . . 49m to produce a horizontally and vertically sub-bandfiltered output signal 48 at the output of the final adder, which signalis stored on a field-by-field basis in the output field store 28. Theoutput sequencer 29B (which can be located, as shown, in the sub-bandfiltering arrangement 12B, though it could be located elsewhere, forexample in the quantizer 14A), is connected to the output field store 28to produce read addresses therefor to cause the data therein to be readout to the quantizer 14A.

It should be noted that the intermediate field store 22 and thetranspose sequencer 24 used in the sub-band filtering arrangement 12A ofFIGS. 2 and 3 are not necessary when the sub-band filtering arrangement12B of FIG. 5 is used. It should however be noted that theabove-described sub-band frequency scrambling that occurs in thesub-band filtering arrangement 12A of FIGS. 2 and 3 also takes place inthe sub-band filtering arrangement 12B of FIG. 5. Thus, the outputsequencer 29B of the sub-band filtering arrangement 12B of FIG. 5 has toperform descrambling.

Before the quantizer 14A is described in more detail, the principle onwhich it operates will be explained with reference to FIGS. 6 and 7.FIG. 6 is a graph representing an empirically determined equationapproximately representing the response of the human psychovisual systemto different spatial frequencies, the vertical axis representing thesensitivity of the human psychovisual system, the horizontal axisrepresenting spatial frequency, and the frequency value fs representingthe Nyquist limit sampling frequency. As can be seen from FIG. 6, thehuman psychovisual system is most sensitive to lower frequencies,peaking at a value just above dc (zero spatial frequency), and thesensitivity rapidly drops as the frequency increases. It is thereforreadily possible for the quantizer 14A to achieve compression of thesub-band filtered video signal by selectively removing information, inconformity with the graph of FIG. 6 (possibly also taking into accountthe amount of aliasing introduced into each sub-band by the sub-bandfiltering), to which the human psychovisual system is effectivelyinsensitive. This is done by quantizing the 64 sub-bands of the sub-bandfiltered video signal by respective appropriate amounts. Specifically,it is assumed that circular symmetry extends the (one-dimensional)response curve of FIG. 6 to two dimensions. (This assumption is believedjustified in that the human psychovisual system is less sensitive todiagonal frequencies than to horizontal and vertical frequencies.) Theresultant generated surface is then integrated under each of the 64sub-band regions to produce an array of 64 numbers (values) which act asthresholds for the purpose of quantization of respective ones of thesub-bands in the quantizer 14A. As will be evident, the numbersdetermine the extent of quantization for their respective sub-bands. If,as in the example described below, the numbers are used to achievequantization by virtue of their being used to divide data arriving fromthe sub-band filtering arrangement 12A or 12B, then the greater thenumber, the greater the quantization threshold and the greater theprobability of a sample in the relevant sub-band having a zero or nearzero value after quantization.

It should be appreciated that the above-described technique ofestablishing the 64 numbers to be used for quantizing the differentsub-bands represents one possible approach only and, even if thisapproach is used, the numbers derived by the somewhat theoretical methoddescribed above may be modified. In more detail, the quality orviewer-acceptability of a picture represented by a video signal whichhas been compressed by the present (or any other) technique andthereafter expanded by a converse technique is, in the final analysis, amatter of subjective opinion. Thus, a final determination of the numbersto used for quantizing the different sub-bands might well best beachieved by selecting rough initial or starting point values by thetheoretical method described above and then refining those initialvalues by viewer testing (trial and error) to produce values judgessubjectively to be optimum.

The above-described 64 numbers can be stored in the form of aquantization matrix (naturally an 8×8 matrix in the case of an 8×8sub-band filtered signal), for example in a look-up table in aprogrammable read only memory (PROM). FIG. 7 shows an example of an 8×8quantization matrix produced for a particular design of sub-bandfiltering arrangement. The positioning of the numbers in the matrix ofFIG. 7 corresponds to the positioning of the sub-bands in FIG. 4. Thatis, for example, the number 68 applies to the dc sub-band and the number8192 applies to the ac sub-band in the bottom right-hand corner in FIG.4. It will be seen that the dc sub-band is only lightly quantized(number=68). Although the two ac sub-bands horizontally and verticallyadjacent to the dc sub-band are quantized marginally even more lightlythan the dc sub-band (number=64), the amount of quantization(quantization threshold) of the dc sub-band is, as can clearly be seenfrom FIG. 7, considerably less than the average of the amounts ofquantization (quantization thresholds) of the ac sub-bands.

The following two factors must be borne in mind concerning thequantization matrix.

(a) The relative values of the numbers, rather than their absolutevalues, are of importance. In this regard, as explained below, thenumbers in the quantization matrix may be scaled before they are used toeffect quantization of the sub-bands in the quantizer 14A.

(b) Since, as mentioned above in the description of FIG. 4, it is beingassumed for the time being that the input digital video signal is acomponent (luminance) signal, rather than a composite signal, thenumbers represented in FIG. 7 apply to a component (luminance) signal.(The modifications made to the quantization matrix of FIG. 7 in the caseof processing a composite signal are explained below.)

In the light of the foregoing explanation of its principle of operation,the quantizer 14A will now be described with reference to FIGS. 8 to 11.

FIG. 8 shows the quantizer 14A in block diagram form. The quantizer 14Acomprises a divider 50 that receives data read thereto from the outputfield store 28 of the sub-band filtering arrangement 12A or 12B underthe control of the output sequencer 29A or 29B, and outputs quantizeddata from the quantizer 14A to the entropy encoder 16A (FIG. 1).

The above-mentioned quantization matrix, referenced 52 in FIG. 8, andstored for example in a look-up table in a PROM, is connected to oneinput of a multiplier 54. A scale factor generator 56 is connected toanother input of the multiplier 54. A sequencer (address generator) 58is connected to the quantization matrix 52 to control it so that itoutputs the appropriate one of the 64 numbers stored in the matrix atthe correct time, that is so that each sample supplied to the quantizeris quantized in accordance with the sub-band in which it is located, andis connected to the entropy encoder 16A to supply thereto a timingsignal that indicates to the entropy encoder whether data being suppliedby the quantizer 14A to the entropy encoder results from quantization ofthe dc sub-band or quantization of the ac sub-bands.

The scale factor generator 56 multiples each of the 64 numbers outputtedby the quantization matrix 52 by a scale factor, whereby the samples ofthe stored field supplied to the quantizer 14A are divided in thedivider 50 by the product of the scale factor and the number currentlyoutputted by the quantization matrix 52. The scale factor is usuallykept constant throughout the period during which the same stored fieldis supplied to the quantizer 14A from the sub-band filtering arrangement12A or 12B, whereby the values for the different sub-band samples asapplied by the multiplier 54 to the divider 50 maintain the samerelationship relative to one another over the field as do the numbers(shown in FIG. 7) in the quantization matrix 52. However, the absolutevalues applied by the multiplier 54 to the divider 50 are determined bythe value of the scale factor. Variation of the scale factor thereforecan vary the output data (bit) rate of the entropy encoder 16A, that isof the entire compression apparatus, and can therefore be employed, forexample, to keep the data rate (which can vary with image content)constant.

The quantizer 14A reads and processes a field of data stored in theoutput field store 28 of the sub-band filtering arrangement 12A or 12B,and passes it on after processing to the entropy encoder 16. Theprocessing comprises, as explained above, and as described in moredetail below, a selective quantization operation used to achievecompression of the video signal. In addition, as explained below, theprocessing involves arrangement of the data outputted to the entropyencoder in a format that readies it for entropy encoding and bit ratereduction.

Since, in the quantizer 14A described above with reference to FIG. 8,the quantization is effected by dividing the input data (in the divider50), the numbers (FIG. 7) in the quantization matrix 52 must be suchthat those for sub-bands that are to be quantized by a relatively largeamount are greater than those for sub-bands that are to be quantized bya relatively small amount. Instead, the quantization could be effectedby multiplying the input data (in a multiplier taking the place of thedivider 50), in which case the numbers in the quantization matrix 52would be such that those for sub-bands that are to be quantized by arelatively large amount are smaller than those for sub-bands that are tobe quantized by a relatively small amount. (For example, in the lattercase the numbers in the quantization matrix 52 could be reciprocals ofthose shown in FIG. 7.) It will be appreciated that, in both cases, theamount of quantization of the dc sub-band is considerably less than theaverage of the amounts of quantization of the ac sub-bands.

FIG. 9 shows a part (the upper left-hand corner) of FIG. 4 on anenlarged scale. More accurately, FIG. 9 is a map of a sub-band filteredfield as supplied to the quantizer 14A from the output field store 28 ofthe sub-band filtering arrangement 12A or 12B, each sub-band beingstored (as mentioned above) in a respective one of an 8×8 array ofregions into which the store 28 can be considered to be partitioned. Inthis regard, the stored field comprises an 8×8 array of sub-bandsfiltered from the corresponding field of the input video signal.

A field of, for example, an NTSC digital video signal has a horizontalextent of 910 samples and a vertical extent of 262 samples. The sub-bandfiltering described above is however carried out on the active part onlyof the field, which part comprises 768 samples in the horizontaldirection and 248 samples in the vertical direction. (In fact, there are243 active samples, corresponding to the number of active lines, in theactive part of an NTSC field. In order to produce numbers of activesamples in both directions that are integrally divisible by 8, 5 blanklines are added to make the number of active samples in the verticaldirection equal to 248.) Thus, each of the 64 sub-band areas in theactive sub-band filtered field comprises (768/8)×(248/8)=2976 samples,that is an array of 96×31 samples (as shown in FIG. 9). (The wholeactive field comprises, of course, 64 times that number of samples.) Theoutput sequencer 29A or 29B of the sub-band filtering arrangement 12A or12B is operative to output the samples of the active field stored in theoutput field store 28 of the sub-band filtering arrangement 12A or 12Bas follows.

The sequencer 29A or 29B first causes all of the 2976 samples formingthe dc sub-band (the upper left-hand sub-band area in FIG. 9), namelythose in that one of the 64 regions of the output store 28 of thesub-band filtering arrangement 12A or 12B containing the dataconstituting that sub-band, to be fed in turn to the quantizer 14A. Thiscan be done by addressing the relevant regions of the output store 28 inan order akin to the raster scan employed to form the full active field,though in this case the area (and the number of samples) is reduced by afactor of 64 as compared to a full field. The process is representedschematically by the arrowed lines drawn in the upper left-hand sub-bandarea in FIG. 9. The resulting 2976 samples are supplied in turn to thedivider 50. While this process is taking place, the sequencer 58 (which,though shown as a separate item, could be combined with the outputsequencer 29A or 29B of the sub-band filtering arrangement 12A or 12B)causes the quantization matrix 52 to output to the multiplier 54 thenumber (68) for the dc sub-band. Thus, all the 2976 samples of the dcsub-band are quantized (by the same amount) by being divided in thedivider 50 by the product of the number (68) for the dc sub-band and thescale factor (from the scale factor generator 56), and passed on as arun or sequence of 2976 samples to the entropy encoder 16A. Also, whilethe above process is taking place, the sequencer 58 causes the timingsignal that it supplies to the entropy encoder 16A to be such as toindicate to the entropy encoder that the quantized samples that it isreceiving relate to the dc sub-band.

When the dc sub-band samples have been processed through the quantizer14A as just described, the sequencer 58 causes the timing signal that itsupplies to the entropy encoder 16A to be such as to indicate to theentropy encoder that the quantized samples that it is about to receiverelate to the ac sub-bands. Thus, the timing signal is changed once perfield; that is, it has a frequency equal to the field frequency. Theoutput sequencer 29A or 29B then causes writing to the quantizer 14A ofthe ac sub-band data, and the sequencer 58 causes a correspondingselection of the numbers to be outputted by the quantization matrix 52,in a manner now to be described.

The ac sub-band data is processed through the quantizer 14A in a ratherdifferent manner than the dc sub-band data. An operation is carried out2976 times, under the control of the output sequencer 29A or 29B, ineach of which the respective 63 samples having a respective one of the2976 spatial positions (pixel sites) in the 63 sub-bands are passed tothe divider and multiplied by their respective coefficients. Thisoperation may be more readily understood by referring to FIG. 9.

In the first of the above-mentioned 2976 operations, as a first step thefirst stored sample accessed is the top left-hand one (indicated by adot) in the ac sub-band numbered 1 in FIG. 9. That sample is divided bythe product of the scale factor and the number in the quantizationmatrix 52 relating to that sub-band, that is the number 64: see FIG. 7.Next as a second step, the same process is repeated for the topleft-hand sample (again indicated by a dot) in the ac sub-band numbered2 in FIG. 9, the number outputted by the quantization matrix 52 in thiscase being the number 64. As a third step, the process is repeated forthe ac sub-band numbered 3 in FIG. 9, the number outputted by thequantization matrix 52 in this case being the number 84. The process isrepeated until it has been carried out 63 times, that is for all of the63 ac sub-bands. The order in which the sub-bands are accessed is inaccordance with the sequence 1 to 63 in which the ac sub-bands aredesignated in FIG. 10 (and, for some only of the ac sub-bands, in FIG.9). It will be seen from FIG. 10 that the order of processing orscanning of the ac sub-bands is a zig-zag order (shown partially byarrowed chain-dotted lines in FIG. 9 for the top left-hand samples) inthat it involves scanning the ac sub-bands in a diagonal direction andin opposite senses. (Thus, the legs of the zig-zag comprise successiveones of a series of groups of the 63 ac sub-bands in a sequence asbetween the groups (legs of the zig-zag) of ac luminance information ofincreasing spatial frequency.) The above-explained zig-zag scanningtechnique is based upon, though considerably modified with respect to, azig-zag scanning technique (described below) that has been proposed aspart of the above-mentioned JPEG (Joint Photographic Experts Group)standard, which (rather than sub-band filtering) requires the use of DCTcoding with 8×8 sample blocks, to each of which an 8×8 DCT transform isapplied, as mentioned at the beginning of this description.

The remaining ones of the above-mentioned 2976 (63-step) operations arecarried out in the same manner as the first one, except that, in eachcase, a respective different one of the 2976 sample sites is used. Thus,for example, in the second operation the samples that are processed arethose having the spatial positions indicated by crosses in FIG. 9, thosebeing those immediately to the right of those, indicated by dots, thatwere processed in the first of the operations.

It will be understood from the foregoing explanation that the datainputted to and outputted by the quantizer 14A for the ac sub-bands(only) has a format as represented in FIG. 11. That is, 2976 successiveseries (hereinafter referred to as "scans")--represented in FIG. 11 byhorizontal strips--of 63 quantized samples are sent to the entropyencoder 16A, each such scan relating to a respective one of the 2976sub-band pixel sites and each such scan having employed the zig-zagtechnique of scanning the 63 ac sub-bands as described above. The totalnumber of samples sent to the entropy encoder 16A per field (includingthe dc sub-band and the ac sub-bands) is the same as the number ofsamples in the stored sub-band filtered field written to the quantizer.However, as will be evident from the foregoing explanation, the datasent to the entropy encoder no longer has any resemblance to a videofield.

During the writing of the dc and ac data from the field store 28 to thequantizer 14A under the control of the sequencer 29A or 29B, thesequencer 58 is operative to control the quantization matrix 52 suchthat each sample supplied to the quantizer is appropriately quantized.Specifically, the matrix 52 first continuously outputs the number (68)for the dc sub-band for a period having a duration of 2976 samples, andthen outputs the 63 numbers for the ac sub-bands in a 63-stagesample-by-sample zig-zag manner corresponding to the manner in which thesamples are written from the field store 28 to the quantizer 14A.

The aim of reducing information in the video field by the quantizingoperation performed in the quantizer 14A, and therefore enablingcompression to be achieved by virtue of the quantizing operation, isachieved by the division operation performed in the divider 50. Thus,particularly for the higher frequency sub-bands, and particularly forimage positions that contain little ac spatial frequency information,the sample outputted by the divider 50 will have a zero or very lowvalue, being constituted wholly or mostly by bits of the value zero. Itshould, however, be noted that, at least in the apparatus presentlybeing described, no reduction in bit (data) rate is carried out in thequantizer 14A. That is, the bit length of each sample outputted by thedivider 50 is the same as that of the sample inputted to it. However,the presence of long runs of zero value samples in the data outputted bythe quantizer 14A, and the reduction in the number of levels to whichthe data inputted thereto can be assigned, enables a consequential bitrate reduction to be effected in the entropy encoder, as describedbelow.

The entropy encoder 16A of the video signal compression apparatus may beembodied in the form shown in FIG. 12. The entropy encoder 16A shown inFIG. 12 complies with a so-called "baseline" version of theabove-mentioned JPEG standard, which version sets out minimalrequirements for complying with the standard, whereby it is in manyrespects of known form or based on known technology and will thereforenot be described in great detail.

The entropy encoder 16A shown in FIG. 12 comprises a switch 60controlled by the above-mentioned timing signal provided to the entropyencoder 16A by the sequencer 58 (FIG. 8) of the quantizer 14A. When thetiming signal indicates that the data emerging from the quantizer 14Arelates to the ac sub-bands, that is when such data is one of the 2976successive scans (each having a length of 63 samples) represented inFIG. 11, the switch 60 directs the data to a run length detector/datamodeller 62. When, on the other hand, the timing signal indicates thatthe data emerging from the quantizer 14 relates to the dc sub-band, thatis when such data is the run or sequence of 2976 samples of the dcsub-band preceding the 2976 successive scans represented in FIG. 11, theswitch 60 directs the data to a differential pulse code modulator (DPCM)64. The switch 60 is thus changed over once per field.

The detector/modeller 62 is connected to a PROM 66 containing a variablelength code (VLC) look-up table and to a PROM 68 containing a fixedlength code (FLC) look-up table. An output of the detector/modeller 62is connected via a multiplexer 70 to the output 18 of the apparatus.

An output of the DPCM 64 is connected to a data modeller 72, an outputof which is in turn connected va the multiplexer 70 to the output 18 ofthe apparatus. In similar manner to the detector/modeller 62, themodeller 72 is connected to a PROM 74 containing a VLC look-up table andto a PROM 76 containing an FLC look-up table. The VLC PROMs shown at 66and 74 may in fact be the same PROM: they are shown as being separate inFIG. 12 largely for the sake of clarity. Similarly the FLC PROMs shownat 68 and 76 may in fact be the same PROM. Further, rather than being(as shown) a separate item, the modeller 72 can be a part (sub-set) ofthe detector/modeller 62.

The operation of the entropy encoder 16A shown in FIG. 12 will now bedescribed, considering first the case in which the data arriving fromthe quantizer 14A relates to the ac sub-bands and is therefore directedby the switch 60 to the detector/modeller 62.

The detector/modeller 62 examines each of the 2976 63-sample scans (FIG.11) arriving from the quantizer 14A and looks for runs of consecutivezero value samples each preceded and followed by a sample of non-zerovalue. The detector/modeller 62 models the incoming data by convertingeach such run of zero consecutive value samples to a word pair of thefollowing form:

    [RUNLENGTH,SIZE][AMPLITUDE].

The two components or "nibbles" (RUNLENGTH and SIZE) of the first wordof the pair each have a length of 4 bits. The bit pattern of the firstnibble (RUNLENGTH) represents in binary form the number of consecutivezero value samples in the run and is generated by a counter (not shown)that counts the number of consecutive zero value samples following aprevious non-zero value. (Run lengths from 0 to 15 are allowed and arunlength continuation is indicated by a code [F,O].) The bit pattern ofthe second nibble (SIZE) represents the number of bits to be used toindicate the amplitude of the sample of non-zero (value) amplitude thatfollows the consecutive run of zero value samples and is looked up fromthe table--represented in FIG. 13--contained in the FLC PROM 68, theleft hand part of FIG. 13 representing ranges of actual values (indecimal form) and the right hand part representing values of SIZE forthe different ranges. The second word (AMPLITUDE) of the pair representsthe amplitude of the sample of non-zero value in the form of a number ofbits determined by the value of SIZE. For a positive non-zero value,AMPLITUDE is the result of truncating the non-zero value (in binaryform) to have only the number of bits specified by SIZE. For a negativenon-zero value, the non-zero value is decremented by one and the sametruncation procedure is followed. To illustrate the nature of the wordpair by way of an example, suppose that the detector/modeller 62 detectsa run of 4 samples of zero value followed by a sample having a value(amplitude) of +7. In this case, the word pair will be as follows:

    [4,3][111].

The number 4 (or, more accurately, its binary equivalent, namely 0100)for RUNLENGTH indicates that the length of the run of zero value samplesis 4. The number 3 (or, more accurately, its binary equivalent, namely0011) for SIZE indicates (as can be seen from FIG. 13) that 3 bits areused to represent the number +7, namely the amplitude (in decimal form)of the sample of non-zero value (amplitude). The number 111 is in factthe amplitude (+7) of the sample of non-zero value expressed in binaryform and truncated to 3 bits.

It will be appreciated that the above operation will be carried out forthe whole of each scan and that a sequence of word pairs will begenerated for each scan. The number of word pairs (that is, the lengthof the sequence of word pairs) generated for each scan will depend uponthe picture content. In general, the greater the number and length ofruns of zero value samples, the lesser the number of word pairs.

The operation of the detector/modeller 62 as so far described representsonly the first of two stages of data (bit) rate reduction carried out inthe detector/modeller. This first stage represents a reduction in bitrate resulting from the above-described reduction of informationeffected in the quantizer 14A that results (without perceptibledegradation in picture content) in a large number of samples of zerovalue (and, more especially, runs thereof) emerging from the quantizer,especially in the data relating to the ac sub-bands.

The second stage of data rate reduction effected in thedetector/modeller 62 is achieved as follows. The first of each of theabove-mentioned word pairs is replaced in the data outputted from thedetector/modeller 62 with a code therefor looked up in the VLC PROM 66.The VLC PROM 66 stores a respective such code for each possible value ofthe first word. The codes are of different lengths, and their lengthsare selected such that the length of each code is, at leastapproximately, inversely proportional to the probability of theassociated word value occurring. In this way, a further reduction in thedata (bit) rate, resulting from entirely loss-free compression, isachieved.

The operation of the entropy encoder 16A shown in FIG. 12 will now bedescribed for the case in which the data arriving from the quantizer 14Arelates to the dc sub-band and is therefore directed by the switch 60 tothe DPCM 64. The dc sub-band (unlike the ac sub-bands) is subjected toDPCM treatment. Since the dc sub-band contains the intensity informationof the original image (field), it has similar statistics to the originalimage. The ac sub-bands, on the other hand, contain sparse image edgeinformation separated by zero value data and thus have completelydifferent statistics to the dc sub-band. Consequently, it is believeddesirable to entropy encode the ac and dc sub-band data separately andin respective different manners to minimize the overall data rate.

Specifically, the dc sub-band data is treated, firstly, in the DPCM 64,prior to entropy encoding proper. The DPCM 64 uses a previous samplepredictor with no quantization of the error data, because the fact thatthe dc sub-band data represents only a small proportion of the overalldata means that high complexity DPCM treatment is difficult to justify.The DPCM 64 decorrelates (adjusts the probability distribution of) thedc sub-band samples so that a greater degree of compression can beachieved in the modeller 72.

Next, entropy encoding proper, resulting in a reduction in the datarate, is carried out in the data modeller 72. The modeller 72 operatessimilarly to the detector/modeller 62, except that there is no detectionof runs of zero value samples, such runs being much less likely in thedc sub-band.

The modeller 72 models the incoming data by converting the incoming datato a sequence of word pairs of the following form:

    [SIZE][AMPLITUDE].

As in the case of the ac sub-band data, SIZE is looked up from the FLCtable of FIG. 13 (in the FLC PROM 76) and indicates the number of bitsused to represent AMPLITUDE. The bits used to represent AMPLITUDE aredetermined in the same way (truncation) as in the case of ac sub-banddata. The word SIZE is the encoded in that it is replaced in the dataoutputted from the modeller 72 with a code therefor looked up in the VLCPROM 74. The VLC PROM 74 stores a respective such code for each possiblevalue of the word. The codes are of different lengths, and their lengthsare selected such that the length of each code is, at leastapproximately, inversely proportional to the probability of theassociated word value occurring. In this way, a further reduction in thedata (bit) rate, resulting from entirely loss-free compression, isachieved.

FIG. 14 is a graph, corresponding to FIG. 4, showing, on thetwo-dimensional frequency plane, what the inventors have discoveredhappens when a field of a digital NTSC composite video signal, sampledat a frequency equal to four times the color sub-carrier frequency fsc(fsc is approximately equal to 358 MHz), is sub-band filtered in a videosignal compression apparatus as described above. The dc and ac luminancedata is distributed among the 64 sub-bands in substantially the same wayas described above for a component (luminance) signal. Surprisingly,however, it was found that the chrominance data, or at least thechrominance data that is needed, is largely (substantially) restrictedto two only of the sub-bands (shown shaded in FIG. 14), namely to thosetwo adjacent sub-bands (hereinafter referred to as "dc chrominancesub-bands") at the bottom centre in FIG. 14. Attempts have been made onan ex post facto basis to explain this phenomenon.

As regards the horizontal positioning of the dc chrominance information,this seems on consideration to be appropriate since it should be centredaround the position pi/2 along the horizontal axis of FIG. 14 by virtueof the use of sampling frequency equal to 4. fsc. Thus, if a samplingfrequency of other than 4. fsc were used, the dc chrominance informationwould be displaced horizontally from the position shown in FIG. 14. Ifthis were the case, the horizontal positioning of the sub-bands to betreated as the dc chrominance sub-bands would differ from that describedabove.

As regards the vertical positioning of the dc chrominance information inFIG. 14, this can be explained as follows. FIG. 15 is a graph showingthe two-dimensional frequency content of a field of an analog NTSCcomposite color video signal, the horizontal axis being in units of MHzand the vertical axis being in units of cycles per picture height (cph).It is of course known that analog NTSC is characterized by a luminancebandwidth of 5.5 MHz and a chrominance bandwidth of 1.3 MHz modulatedabout the color sub-carrier frequency of 3.58 MHz. It is also known thatthe number of sub-carrier cycles per line is 227.5, as a result of whichthe phase of the sub-carrier is shifted by 180 degrees for each line.This is responsible for a modulation of the chrominance signalvertically, which, as shown in FIG. 15, leads to the chrominance beingcentered at a spectral position of 131.25 cph. This appears to explainthe vertical positioning of the chrominance information in FIG. 14.Thus, the process of modulation generates lower and upper sidebands.Since the vertical carrier frequency is at the Nyquist limit frequency,the upper sidebands are on the other side of the Nyquist limit and thusdo not form part of the frequency plane of FIG. 14. Therefore, for NTSC,the dc chrominance data will appear at the bottom of FIG. 14.

As regards the horizontal extent of the dc chrominance information, thefairly harsh filtering (horizontal bandwidth restriction) to which thecolor (chrominance) information is subjected before it is modulated ontothe luminance information appears to explain why the horizontal extentof the chrominance is restricted as shown in FIG. 14, namely so that itfalls largely within two horizontally adjacent ones of the 64 sub-bandsemployed in this case, that is so that the horizontal extent is equal toabout pi/4. (In fact, as explained below, the dc chrominance data infact "spills over" somewhat into the two sub-bands in the bottom row ofFIG. 14 that are horizontally adjacent to those shown shaded.)

It seems on reflection that the vertical extent of the needed colorinformation in FIG. 14 is restricted to about the height of one of thesub-bands, namely about pi/8, for the following reason. It is probablethat the dc chrominance information is wholly or largely restricted tothe two sub-bands shown shaded at the bottom of FIG. 14. It is likewiseprobable that ac chrominance appears in at least some of those sub-bandsabove the two shown shaded at the bottom of FIG. 14. However, since thehuman psychovisual system has a low sensitivity to high frequency (ac)chrominance information, it appears to produce subjectively acceptableresults if any such sub-bands that are co-occupied by ac luminance andac chrominance information are treated as if they are occupied only byac luminance information.

However, whatever the explanation, the restricted bandwidth (in bothdirections) of the needed color information has proven very fortunatebecause, as is explained below, it leads to the advantageous effectthat, with very minor modification, the apparatus as described above canhandle an NTSC composite color video signal. Thus, conversion of thesignal to component form, and tripling of the hardware to handle thethree components separately, is not necessary, leading to a large savingin expense.

The only modification that has to be made to the apparatus as describedabove to enable it to handle an NTSC color composite signal is to changethe numbers in the quantization matrix 52 that determine the amount ofquantization of the sub-bands that contain the dc chrominance data,namely the two dc chrominance sub-bands as shown shaded in FIG. 14.Specifically, instead of being heavily quantized as high frequency acluminance sub-bands of relatively little importance, the two sub-bandsshould be relatively lightly quantized so as to preserve the dcchrominance information. The amount of quantization is in fact desirablyreduced to about the same level as applied to the dc luminance sub-band.The necessary effect can therefore be achieved by changing the twobottom center numbers in the quantization matrix as represented in FIG.7 from their values of 1856 and 2491, for a component (luminance)signal, to 68 (or thereabouts) for an NTSC composite signal. This isshown schematically in FIG. 7.

In principle, no changes other than the above-described change to twonumbers in the quantization matrix 52 are necessary to enable theapparatus to handle a digital NTSC composite color video signal. Inparticular, it is to be noted that the (now lightly quantized) dcchrominance sub-bands can be handled in the quantizer 14A and entropyencoder 16A together with, and in the same manner as, the ac luminancesub-bands.

Although, in principle, only the above-described change in thequantization is necessary to enable the apparatus to handle a digitalNTSC color composite signal, another change that can advantageously bemade is as follows. The zig-zag sequence or order in which, for acomponent (luminance) signal, the 63 sub-bands other than the dcluminance sub-band are quantized and then entropy encoded is, asexplained above, shown in FIG. 10. It will be seen that, in the case ofa digital NTSC color composite signal, the dc chrominance sub-bands havethe positions 49 and 57 in the sequence. This could result in a decreasein the efficiency of compression in that the dc chrominance sub-bandsare much more likely than the adjacent sub-bands in the sequence tocontain non-zero value samples: that is, they could break up runs ofzero value samples. (This is even more likely in the case of PAL thanNTSC because, as explained below, in the case of PAL there are four dcchrominance sub-bands positioned in the center of the frequency plane asshown in FIG. 14.) Thus, preferably, the apparatus is further modifiedin that the sequencer 29A (or 29B) is modified to change the zig-zagsequence so that the dc chrominance sub-bands occupy (in any specifiedorder) the first positions in the sequence and the remaining sub-bandsoccupy the remaining positions in the sequence in the same order asbefore. That is, in the case of an NTSC signal, and using the samenumbering system for the sub-bands as shown in FIG. 10, the sequencewill comprise, in the following order, sub-band 49 (or 57), sub-band 57(or 49), sub-bands 1 to 48, sub-bands 50 to 56, and sub-bands 58 to 63.(The changed sequence that would be adopted in the case of a PAL signal,as will be clear from the description given below with reference to FIG.17, will be sub-bands 24, 31, 32 and 39 (in any order), sub-bands 1 to23, sub-bands 25 to 30, sub-bands 33 to 38, and sub-bands 40 to 63.) Thesequencer 58 in the quantizer 14A (if separate from the sequencer 29A or29B) is modified in correspondence with the way in which the sequencer29A or 29B is modified in order to ensure that each sub-band isappropriately quantized. That is, instead of outputting the 63 numbersfor the sub-bands other than the dc luminance sub-band as shown in FIG.7 in the same zig-zag order as that in which the sub-bands other thanthe dc luminance sub-band are numbered 1 to 63 in FIG. 10, the sequencer58 is modified so that it outputs those numbers in an order which ismodified in the same way in which the zig-zag sequence of quantizing thesub-band filtered samples is (as was just explained) modified.

Further consideration was given to the phenomenon of spectralconcentration of the color information by examining the two-dimensionalfrequency plane for a frame (as opposed to a field) of a digital NTSCcomposite color video signal sampled at 4. fsc, as shown in FIG. 16. Itwill be seen that the composite data in the center of the frequencyplane is composed of four distinct regions due to modulation of thenegative frequencies. These four regions are identical except forfrequency inversion and a phase shift. Ideally, as explained below, thechrominance data should be restricted to a small number of thesub-bands. FIG. 16 indicates that the use of 64 (8×8) sub-bands is agood choice in this respect.

Ideally, the horizontal extent or span of the sub-bands should equal thebaseband chrominance bandwidth for efficient compression. This isbecause, in this case, the chrominance information falls exactly withinthe relevant sub-bands, that is it occupies the whole of those sub-bandsand does not occupy parts of adjacent sub-bands, so that all of the dcchrominance information is lightly quantized and no substantial amountof adjacent ac luminance information is lightly quantized. In otherwords, a smaller span would lead to the chrominance data falling into agreater number of sub-bands (which is in conflict with theabove-mentioned requirement of keeping the number of chrominancesub-bands as small as possible) and a greater span would lead to theadjacent luminance data not being appropriately quantized.

It will be seen from FIG. 16 that there is in fact a small overlap or"spill over" of chrominance data into adjacent sub-bands which aretreated as ac luminance sub-bands, whereby the overlapping parts of thechrominance will be (heavily) quantized in accordance with thequantization thresholds set for those adjacent sub-bands. In practice,it is believed that the results will nonetheless be subjectivelyacceptable. The overlap occurs in the horizontal direction because, ascan be seen from FIG. 16, the horizontal extent of each sub-band isapproximately equal to 0.9 MHz, whereas the chrominance data has abandwidth (two sidebands) of 1.3 MHz, which is slightly larger.Provided, of course, that the overlap is not so large that a significantamount of low-frequency chrominance information spills over intoadjacent sub-bands which are treated in the quantization process as acluminance sub-bands, the overlap will generally be tolerable because, asexplained above, it will comprise higher frequency chrominanceinformation to which the human psychovisual system is not verysensitive. However, the overlap could be avoided, in theory, by slightlyincreasing the size of the sub-bands in either or both directions, thatis by slightly decreasing the total number of sub-bands. Thus, aninspection of FIG. 16 indicates that the overlap would be reduced if a7×7 or a 6×6 array were used. While such an array is realizable intheory, it could not be realized in the case of the "tree" or"hierarchical" QMF structure described with reference to FIGS. 2 and 3because this can only produce, in each direction, a number of sub-bandswhich is an integral power of two. Thus, if the tree structure is to beused, the overlap described above could be avoided only by going down toa 4×4 array. While a 4×4 array is usable and produces acceptableresults, it would result in the extent of the sub-bands that would haveto be used as chrominance sub-bands (which, similarly to FIG. 14, wouldbe the two at the bottom center of the 4×4 array) being substantiallygreater than the extent of the dc chrominance data. Also, it wouldreduce the efficiency of compression by virtue of the fact that thenumber of sub-bands would be greatly reduced. The reason for this is asfollows.

The amount of compression achievable by virtue of the quantization stepdecreases, up to a certain extent, as the number of sub-bands decreases.This is because the ratio between the number of ac luminance sub-bandsand the number of dc (luminance and chrominance) sub-bands will increasewith the total number of sub-bands and the ac sub-bands are on averagemore heavily quantized than the dc sub-bands. Thus, for example, inabove-described case in which there are 64 sub-bands, of which one is adc luminance sub-band and two (for NTSC)--or four (for PAL, seebelow)--are dc chrominance sub-bands, either 61 (for NTSC)--or 59 (forPAL)--of the 64 sub-bands are ac luminance sub-bands. That is, either61/64 or 59/64 of a field can be relatively heavily quantized onaverage, thereby enabling a higher degree of compression to be achievedthan would be the case if the number of sub-bands were less than 64.(Thus, for example, if 16 (4×4) sub-bands were used, only 13/16 of afield (for NTSC) would be ac luminance sub-bands.) Therefore, it is ingeneral desirable to use as large a number of sub-bands as is practical,bearing in mind, however, that hardware realization will becomeimpractical if too many sub-bands are used. Also, if a large increase(over an 8×8 array) is made in the number of sub-bands, there will be nonet benefit (or at least not a greatly increased benefit) because morethan two of the sub-bands (for NTSC) or more than four of the sub-bands(for PAL) may have to be treated (due to extensive overspill ofchrominance information) as dc chrominance sub-bands. At present, theuse of an 8×8 square array (or a non-square array of similar size) isbelieved to provide a good compromise between the above constraints,though, as mentioned above, a 4×4 array is usable. Also arrays havinghorizontal and vertical extents of 4 and 8, and 8 and 4, respectively,are usable, the latter being considered promising. At the very least, itis highly preferable for the number of ac luminance sub-bands to exceedthe number of dc luminance and chrominance sub-bands.

As an alternative to ignoring limited overspill or increasing the sizeof the sub-bands to reduce or remove overspill, it is possible to takeaccount of the fact that some chrominance information appears in bandsadjacent to these treated (in the quantization operation) as dcchrominance sub-bands by quantizing the adjacent sub-bands to an extentintermediate that to which they would be quantized if considered as acluminance sub-bands only, and that to which the sub-bands treated as dcchrominance sub-bands are quantized. The actual extent of quantizationof the adjacent sub-bands might well have to be established empirically.

As mentioned above, the use of sampling frequency equal to four timesthe color sub-carrier frequency is preferred since it has the effect ofcentering the dc chrominance sub-bands about pi/2 in the horizontaldirection, that is locating them in the horizontal sense where shown inFIG. 14. However, other sampling frequencies can be used.

The foregoing description with reference to FIGS. 14 to 17 hasconcentrated on NTSC composite color video signals. It is to be noted,however, that the technique outlined above can be applied to otherbroadcast standard composite color video signals. The application of thetechnique to PAL composite color video signals will now be described.

FIG. 17 is a view corresponding to FIG. 4, but showing on thetwo-dimensional frequency plane both the sub-band filtered field of anNTSC composite color video signal, and a sub-band filtered field of aPAL composite color video signal, each sampled at four times its colorsub-carrier frequency. It will be seen that, in the case of PAL, thechrominance information occupies (in the case of an 8×8 array ofsub-bands) the four sub-bands (shown shaded) clustered at the center,rather than, as in the case of NTSC, the two at the bottom center,namely those numbered 24, 31, 32 and 39 in FIG. 10.

The only modification that has to be made to the apparatus as describedabove to enable it to handle a PAL color composite signal is to changethe numbers in the quantization matrix 52 that determine the amount ofquantization of the sub-bands that contain the chrominance data in thecase of PAL, namely the four PAL dc chrominance sub-bands as shownshaded in the center of FIG. 17. Specifically, instead of being heavilyquantized as high frequency ac luminance sub-bands of relatively littleimportance, the four sub-bands should be relatively lightly quantized soas to preserve the dc chrominance information. As in the case of NTSC,for PAL also the amount of quantization is in fact desirably reduced toabout the same level as applied to the dc luminance sub-band. Thenecessary effect can therefore be achieved by changing the four numbersclustered in the center of the quantization matrix as represented inFIG. 7 from their values of 260,396,396 and 581, for a component(luminance) signal, to 68 for a PAL composite signal. This is shownschematically in FIG. 7.

Further, in the case of PAL also, the apparatus is desirably furthermodified (as already indicated above) to change the zig-zag sequence oftreatment of the 63 sub-bands other than the dc luminance sub-band sothat the four dc chrominance sub-bands come first.

Since, in the case of PAL, the chrominance data occupies 4 of the 64sub-bands, whereas in the case of NTSC the chrominance data occupiesonly 2 of the 64 sub-bands, there is a slightly lower potential forcompression (as compared to NTSC) for PAL. Specifically, as indicatedabove, only 59/64 of a field in the case of PAL, as opposed to 61/64 ofa field in the case of NTSC, is occupied by ac luminance sub-bands andtherefore can be relatively heavily quantized on average.

The invention can, of course, be embodied in other ways than thatdescribed above by way of example. For instance, although theabove-described apparatus operates on a field-by-field basis, which willgenerally be more convenient, it could instead operate on aframe-by-frame basis. In this case the sub-bands would have twice thenumber of samples in the vertical direction and the various field storeswould be replaced by frame stores.

Further, although the above-described apparatus operates only on anintra-field basis, whereby sub-band filtering is effected in twodimensions or directions only, namely the horizontal and verticalspatial directions, it could in principle be extended to operate also onan inter-field or inter-frame basis, whereby sub-band filtering would inthis case be effected in three dimensions or directions, namely thehorizontal and vertical spatial directions and the temporal dimension ordirection.

As alluded to in the introduction to this specification, the format(described above with reference to FIGS. 9 to 11) of the data writtenfrom the field store 28 to the quantizer 14A, under the control of thesequencer 29A or 29B, for quantization and entropy encoding, and theconsequential outputting of the numbers of the quantization matrix 52(FIG. 7), under the control of the sequencer 58, are very different thanin the case of the JPEG (DCT) standard. The same applies to the timingsignal supplied to the switch 60 of the entropy encoder 16A by thesequencer 58, in that the entropy encoder has in the above case to beswitched at the field or frame frequency rather than at the much higherfrequency (block frequency) used in the case of the JPEG (DCT) standard.While this form of sequencing is believed superior to the JPEG sequenceat least in some cases, in that it groups the dc and ac informationtogether rather than intermingles it, it is subject to the disadvantagethat difficulty might arise if the compression apparatus must operate inclose conformity with the JPEG standard and/or if there is a desire toconstruct the apparatus using a quantizer 14A and/or an entropy encoder16A designed specifically for use in accordance with the JPEG standard.As will now be described, this difficulty can be overcome by modifyingthe operation as described above to operate in close accordance with theJPEG standard. The modification is possible by virtue of a realizationof an extent of commonality between decorrelation effected by thetechnique of sub-band filtering and decorrelation effected by way of thevery different technique of linear block transformation, for example byway of a DCT technique. The commonality will now be explained,commencing with a brief review of the linear block transformationmethod.

FIG. 18 shows how the samples making up a field or frame of a digitalvideo signal are divided into blocks or arrays of samples which are eachto be processed by a linear transform, for example a DCT. It is assumed,by way of example, that the item depicted in FIG. 18 is a frame of a4:2:2 component (luminance or color difference) signal according to CCIRRecommendation 601. If DCT were employed in a compression apparatus likethose embodying the invention as described above, it might instead bethe case that a field (or frame) of an NTSC or PAL composite signalwould instead in fact be processed. The only difference in that case isthat there would be a different number of blocks since a field (orframe) of an NTSC or PAL composite signal has a different extent (numberof samples) in both the horizontal and vertical directions than a CCIR601 4:2:2 frame.

The frame shown in FIG. 18 has a horizontal extent of 720 samples and avertical extent of 576 samples. Prior to being processed by a lineartransform, for example a DCT, the frame is divided by suitable hardwareinto blocks BL of (for example) 8×8 samples. Since 720 and 576 are eachintegrally divisable by 8, the frame is divided into an array of(720/8×576/8=) 6480 blocks, the array having a horizontal extent of(720/8=) 90 blocks and a vertical extent of (576/8=) 72 blocks.

FIG. 19 shows a linear transform decorrelator 12C for carrying out theabove-outlined operation. A digital input video signal is applied via aninput 10 to a blocking circuit 80 that divides each field or frame ofthe signal into 8×8 sample blocks. In a manner analogous to a rasterscan, the blocking circuit 80 sequentially outputs the blocks to alinear transform circuit 82 which transforms each block. Forconvenience, it will be assumed that the linear transform circuit 82performs a DCT transform; and the circuit will thus hereinafter bereferred to as a DCT circuit. However, as indicated above, othersuitable linear block transforms known in the art can be used.

The transformation performed by the DCT circuit 82 on each 8×8 block BLof samples results in the circuit outputting an 8×8 block BL(T) oftransformed samples which are (somewhat confusingly) referred to in theart as "coefficients". Each coefficient is a sample or measure of thefrequency content of the video signal at a respective one of an 8×8array of positions in the two-dimensional frequency domain or planecorresponding to a respective one of the samples inputted to the DCTcircuit 82. The coefficient blocks BL(T) are supplied from the DCTcircuit 82 to a quantizer 14A and entropy encoder 16A for compression ofthe signal, as described above with reference to FIG. 1. As explainedbelow, the decorrelator 12C causes the coefficients to be supplied tothe quantizer 14A in a rather different manner than that in which thesequencers 29A and 29B of FIGS. 2 and 5, respectively, cause thesub-band filtered samples in the stores 28 of FIGS. 2 and 5 to bewritten to the quantizer.

FIG. 20 shows a DCT decorrelator 12D that employs DCT correlationfollowed by a coefficient reordering process that emulates sub-bandfiltering. The decorrelator 12D of FIG. 20 is essentially the same asthe decorrelator 12C of FIG. 19, except that: (i) the DCT circuit 82 isfollowed by a reorder circuit (address generator) 83 followed by a store84, the reorder circuit 83 being operative, as described below, to causethe coefficients making up the blocks BL(T) emerging from the DCTcircuit 82 to be written into the store 84 in a very different manner tothat in which they are outputted from the DCT circuit 82 in the case ofFIG. 19; and (ii) the writing of data from the store 84 to the quantizer14A is controlled by an output sequencer (address generator) 29D, whichcan operate similarly to the output sequencer 29A (29B) of FIG. 2 (FIG.5).

In the case of FIG. 20, the store 84 can be considered to be partitionedinto a number of regions equal to the number of coefficients per block(64, that is 8×8, in the above example), each such region having acapacity equal to the number of blocks (6480, that is 90×72, in theabove example). In this case, the 64 coefficients making up eachcoefficient block BL(T) are spread out over the whole of the store 84rather than being outputted as a unit to the quantizer 14A as in thecase of FIG. 19. More specifically, each of the 64 coefficients makingup each coefficient block BL(T) is written into a respective one of the64 regions into which the store is partitioned. The exact positioning ofeach coefficient within its respective region of the store 84 will nowbe explained with reference to FIGS. 21 and 22.

FIG. 21 shows one of the 8×8 coefficient blocks BL(T). FIG. 22 shows thestore 84 of FIG. 20 partitioned, as mentioned above, into 64 (8×8)regions R each having a capacity equal to 6480 (90×72) coefficients. Theway in which each coefficient is positioned by the reorder circuit 83 inthe store 84 is as follows. Assume that the coefficient block BL(T) isthat corresponding to the first input sample block BL, namely that shownin the upper left-hand corner of FIG. 18. Employing a convention inwhich the coefficients in the block (BL(T) are identified as c(m,n),where m varies from 0 to 7 and represents the horizontal position of thecoefficient within the block and n varies from 0 to 7 and represents thevertical position of the coefficient within the block, the origin beingthe coefficient c(0,0) in the upper left-hand corner in FIG. 21, andemploying an identical convention to identify the regions R(m,n) of thestore 84 as shown in FIG. 22, each coefficient c(m,n) of the coefficientblock BL(T) corresponding to the input sample block BL shown in theupper left-hand corner of FIG. 18 is store din the upper left-hand oneof the 90×72 array of storage positions in that one of the regions Ridentified by the same values of m and n as the coefficient. Thesecoefficients are thus stored in positions represented (for some only ofthe regions R) by dots in FIG. 22.

A similar process is then carried out for the coefficients c(m,n) of thesecond coefficient block BL(T), namely that corresponding to the inputsample block BL which is horizontally adjacent to and on the right ofthe input sample block shown in the upper left-hand corner of FIG. 18.The coefficients of the second block BL(T) are stored in the next set ofstorage positions in the regions R of the store 84, namely thosehorizontally adjacent to and on the right of the positions in which thecoefficients of the first block (BL(T) were stored. The coefficients ofthe second block BL(T) are thus stored in positions represented (forsome only of the regions R) by crosses in FIG. 22.

The foregoing process is then repeated, in a manner analogous to araster scan, for the coefficients c(m,n) of each of the remainingcoefficient blocks (BL(T), until the coefficients of the final blockBL(T), namely that corresponding to the input sample block BL which isshown in the bottom right-hand corner of FIG. 18, are stored in thefinal set of storage positions in the regions R of the store 84, namelythose in the bottom right-hand corners of the regions R. That is, thecoefficients of the final block BL(T) are stored in positionsrepresented (for some only of the regions R) by circles in FIG. 22.

In more general terms, employing a convention in which the coefficientblocks BL(T) are identified as BL(T)(p,q), where p varies from 0 to 90and represents the horizontal position of the block and q varies from 0to 72 and represents the vertical position of the block, the originbeing the coefficient block corresponding to the input sample block BLin the upper left-hand corner in FIG. 18, and employing an identicalconvention to identify the storage positions s(p,q) of each of theregions R(m,n) of the store 84, each coefficient c(m,n) of eachcoefficient block BL(T)(p,q) is stored in that one of the regions Ridentified by the same values of m and n as the coefficient and, withinthat region, in that one of the storage position s of that region havingthe same values of p and q as the coefficient block.

If the structure of the data content of the store 84 as represented inFIG. 22 is analyzed, it will be seen that, starting from the regionR(0,0) in the upper left-hand corner, the content of each regionincreases in horizontal spatial frequency as one goes right(horizontally) and increases in vertical spatial frequency as one goesdown (vertically). That is, for example, the region R(0,0) will containdc spatial frequency information (that is, the coefficients c(0,0) ofall of the blocks BL(T)), the region R(0,7) will contain the highestvertical frequency information and dc horizontal frequency information,the region R(7,0) will contain the highest horizontal frequencyinformation and dc vertical frequency information, and the region R(7,7)will contain the highest diagonal frequency information. Thus, thereordering process effected by the reorder circuit 83 results in thecontent of the store 84 being such that the contents of the differentregions R thereof are data sets which are in substance the same as wouldhave been obtained if, instead of being decorrelated in the decorrelator12D of FIG. 20, the video signal had been decorrelated in a decorrelatorin the form of a sub-band filtering arrangement, for example either thearrangement 12A described above with reference to FIGS. 2 and 3 or thearrangement 12B described above with reference to FIG. 5. That is, thecontent of the store 84 of the decorrelator 12D of FIG. 20 as read outto the quantizer 14A is substantially the same as the contents of thestores 28 of the decorrelators (sub-band filtering arrangements) 12A and12B of FIGS. 2 and 3, and FIG. 5, respectively, as read out to thequantizer 14A.

Thus, the decorrelator 12D of FIG. 20 emulates the sub-band filteringcarried out in the decorrelators (sub-band filtering arrangements) 12Aand 12B, as a consequence of which the decorrelator 12D could (thoughthis is not done in the present invention, since the emulated sub-bandfiltering technique does not preserve the advantages of actual sub-bandfiltering over the DCT approach) be used in direct substitution for thedecorrelator 12A or 12B in a video signal compression signal apparatuswhich can handle a digital composite color video signal (or a componentvideo signal). In this regard, assuming that the decorrelator 12D isconfigured to process, for example, an NTSC signal on a field-by-fieldbasis, the content of the store 84 will correspond to FIG. 14. Thus, theregion R(0,0) of the store 84 will be quantized in the quantizer 14A (asdescribed above with reference to FIGS. 7 and 8) on the basis that itcontains dc luminance information, the contents of the regions R(3,7)and R(4,7) will be quantized on the basis that they contain dcchrominance information, and the contents of the other 61 regions willbe quantized on the basis that they contain ac luminance information.Likewise, if the decorrelator 12D is configured to process a PAL signalon a field-by-field basis, the content of the store 82 will correspondto the relevant parts of FIG. 17. Thus, the region R(0,0) of the store84 will be quantized on the basis that it contains dc luminanceinformation, the contents of the regions R(3,3), R(3,4), R(4,3) andR(4,4) will be quantized on the basis that they contain dc chrominanceinformation, and the contents of the other 59 regions will be quantizedon the basis that they contain ac luminance information.

The foregoing description with reference to FIGS. 18 and 22substantiates the above suggestion of a commonality or duality betweensub-band filtering and linear transformation in that the two-dimensionalspatial frequency information obtained in the case of the former ispresent also in the coefficients obtained in the case of the latter andcan be recovered by data reordering. Thus, it is possible to compress adigital composite color video signal (without splitting the compositesignal into its components), not only by using sub-band filtering fordecorrelation, but also by emulating sub-band filtering by using lineartransform (for example, DCT) decorrelation followed by data reordering.However, as will now be described, realization of the commonality leadsto the further development that merely by altering the operation of thesequencer 29A (or 29B) of the decorrelator 12A (or 12B) of FIGS. 2 and 3(or FIG. 5), and correspondently modifying the operation of thesequencer 58 of the quantizer 14A, a video signal compression apparatusembodying the invention, in which the data format outputted by thedecorrelator 12A (or 12B) is substantially the same as that of the JPEGstandard, can be achieved.

In the foregoing regard, in the apparatus described above with referenceto FIGS. 2 to 17, the sub-bands stored in respective ones of the 64regions into which the output store 28 of the decorrelator 12A or 12B ispartitioned comprise respective data sets representing dc luminanceinformation, ac luminance information and (if a composite color videosignal is being compressed) dc chrominance information of the videosignal in the two-dimensional frequency domain. It was demonstratedabove that there is a commonality or duality between sub-band filteringand transform decorrelation in that, in the case of transformdecorrelation as described with reference to FIGS. 18 and 19, the datasets obtained in the case of sub-band filtering are still present (inthat each coefficient block BL(T) contains a respective member of eachof the data sets) and the data sets can be put into storage (in thestore 84 of FIG. 20) in the same manner as in the case of sub-bandfiltering so as thereby to emulate sub-band filtering whereby the storeddata sets can be treated after outputting from the store 84 insubstantially exactly the same way as if they had been obtained bysub-band filtering. Pursing the commanality further, it is in fact thecase that, since the data sets obtained in the case of sub-bandfiltering are still present in the case of transform decorrelation, itis possible to reorder the data outputted by the decorrelator 12A or 12Bto conform to the format obtained by the use of transform coding per se,that is as would be obtained if the DCT decorrelator 12C of FIG. 19 wereused.

This is accomplished as follows. Instead of the sequencer 29A or 29Bbeing operative (as described above with reference to FIGS. 9 to 11) tofirst scan or output all of the 2976 samples (for NTSC) of the storageregion of the store 28 holding the dc sub-band and then to zig-zag scanthe storage regions holding the remaining 63 sub-bands (each made up of2976 samples) 2976 times, each time scanning 63 of the samples having acommon one of the 2976 possible spatial positions, the sequencer 29A or29B zig-zag scans all 64 of the storage regions (each made up of 2976samples) 2976 times, each time scanning all 64 of the samples having acommon one of the 2976 possible spatial positions. That is, theoperation of the sequencer 29A or 29B is modified with respect to thatdescribed with reference to FIGS. 9 to 11 in that the samples in thefield store 28 are outputted by zig-zag scanning them in an order whichis the same as that shown in FIG. 10, save that the order in the presentcase is a 64-stage one (rather than a 63-stage one) starting with thearea in FIG. 10 that is not numbered and then carrying on in the orderof the areas numbered 1 to 63. The 64-stage zig-zag would be as shown byarrowed chain-dotted lines in FIG. 21 if the dots in FIG. 21 wereconsidered to represent (instead of the coefficients of a transformedcoefficient block BL(T) in the DCT decorrelator 12D of FIG. 20) thosepixels represented by dots in FIG. 9.

The data that in this case is inputted to and outputted from thequantizer 14A has a very different form than that described withreference to FIG. 11. Instead of there being a run of 2976 samplesrelating to the dc sub-band followed (as shown in FIG. 11) by 2976 scansor sequences of 63 samples (one for each ac sub-band) each relating to arespective one of the 2976 sub-band spatial positions, there are 2976successive scans or sequences of 64 samples (one for each of the 64sub-bands) each relating to a respective one of the 2976 sub-bandspatial positions. It is therefore necessary for the sequencer 58 of thequantizer 14A to operate in a correspondingly different manner. That is,instead of first outputting the same number from the quantization matrix52 as shown in FIG. 7 (that for the dc sub-band) continuously for aperiod having a duration of 2976 samples, and then cyclically outputtingthe 63 numbers for the other sub-bands, 2976 times, in a 63-stage,sample-by-sample zig-zag manner, as described above, so as to conformwith the manner of reading the output field store 28 as described above,in this case the sequencer 58 always cyclically outputs all 64 numbersof the quantization matrix 52 in a zig-zag manner which is the same asthe 64-stage zig-zag scanning of the 64 samples of the same spatialposition in the 64 regions of the store 28 containing the respectivesub-bands.

Also, the timing signal supplied to the entropy encoder 16A of FIG. 13by the sequencer 58 must be altered to reflect the fact that there is adifference in the timing of receipt by the entropy encoder of datarelating to dc frequency information (which is switched by the switch 60to the DPCM 64) and ac frequency information (which is switched by theswitch 60 to the run length detector/data modeller 62). As describedabove with reference to FIGS. 9 to 11, in that case the switch 60switches the data to the DPCM 64 for the initial run of samples(duration of 2976 samples, in the case of NTSC) of the dc sub-band, andthen switches the data to the detector/modeller 62 for the other 63sub-bands (duration of 63×2976 samples). That is, the switch 60 ischanged over once per field (or frame). In the present case, thesequencer 58 must be operative to change over the switch 60 rather morefrequently. Specifically, the switch 60 is changed over every 64samples, that is once per 64-stage zig-zag scan, to supply the firstsample of each scan (namely that in the region of the store 28containing the dc sub-band), after quantization, to the DPCM 64, and tosupply the remaining 63 samples of each zig-zag scan (namely those inthe other 63 storage regions of the store 28), after quantization, tothe detector/modeller 62. In short, while the switch 60 has to bechanged over (by the sequencer 58) once per field (or frame) in the caseof FIGS. 9 to 11, in the present case the switch 60 has to be changedover every 64 samples, that is once every 64-stage zig-zag scan.

Thus, in summary of the above-described modification, the format of thedata to be quantized is very similar to that in the case of the JPEG(DCT) standard, which has the advantage that quantization can besequenced in a very similar or even identical way to that used in thecase of the standard. Thus, it may be possible to use an "off the shelf"chip or assembly intended for use in a JPEG compression apparatus(possibly with changes in the quantization values, that is the numbersin the quantization matrix 52 as shown in FIG. 7) for the quantizer 14A.Also, the entropy encoder 16A is switched at the frequency of carryingout the 64-stage zig-zag scanning operations in which those samplescorresponding to a respective one of the spatial positions in each ofthe store data sets are quantized, that is at a frequency determined bythe number of data sets (sub-bands), rather than at the field or framefrequency. If, as described, the number of data sets (sub-bands) is thesame (8×8=64) as the number of samples per block as specified in theJPEG standard, the frequency of switching the entropy encoding is thesame as the frequency (block frequency) used in the case of the JPEG(DCT) standard. Thus, it may be possible to use an "off the shelf" chipor assembly intended for use in a JPEG compression apparatus for theentropy encoder 16A.

Although illustrative embodiments of the invention have been describedin detail herein with reference to the accompanying drawings, it is tobe understood that the invention is not limited to those preciseembodiments, and that various changes and modifications can be effectedtherein by one skilled in the art without departing from the scope andspirit of the invention as defined by the appended claims.

We claim:
 1. A method of compressing a video signal, the methodcomprising the steps of:effecting spatial two-dimensional sub-bandfiltering of a digital video signal to form a plurality of data setsconstituting respective sub-bands of the two-dimensional spatialfrequency domain; storing said data sets for a field or frame of saidvideo signal; quantizing the stored data sets in accordance withrespective values, said values being such that the amount ofquantization of one of the data sets constituting a sub-band to which dcluminance information of the signal is at least predominantly confinedis less than the average of the amounts of quantization of the remainingdata sets; writing the stored data to a quantizer, for carrying out saidquantizing, in a desired sequence; controlling said quantizing inaccordance with said desired sequence such that each datum written tosaid quantizer is appropriately quantized; entropy encoding thequantized data sets such that quantized data representative of dcluminance information is coded by a first coding technique and quantizeddata representative of ac luminance information is coded by a secondcoding technique; and controlling said entropy encoding in accordancewith said desired sequence such that each quantized datum is subjectedto the appropriate one of said first and second coding techniques.
 2. Amethod according to claim 1, wherein said sub-bands make up, in thetwo-dimensional spatial frequency domain, a rectangular array having adimension of 8 in the direction of scanning of the video signal and adimension of 4 in the direction orthogonal thereto.
 3. A methodaccording to claim 1, wherein:said desired sequence in which the storeddata is written to said quantizer is such that, upon each occurrence ofan operation carried out for a number of times equal to the number ofdata in each stored data set, those data corresponding to a respectiveone of the spatial positions in each of the stored data sets arequantized in a predetermined order; said quantizing is controlled suchthat each of the data written to said quantizer in each of saidoperations is quantized in an amount appropriate to the data set ofwhich it forms a part; and said entropy encoding is controlled suchthat, for each of said operations, that one of the quantized dataforming part of the data set constituting said sub-band to which the dcluminance information is at least predominantly confined is subjected tosaid first coding technique and all of the other data are subjected tosaid second coding technique.
 4. A method according to claim 3, whereinsaid predetermined order comprises successive groups of the stored datasets in a sequence, as between said groups, of sub-bands containing acluminance information of increasing spatial frequency.
 5. A methodaccording to claim 3, wherein:said digital video signal is a digitalcomposite color video signal, the stored data sets are quantized inaccordance with respective values which are such that the amounts ofquantization of each of said one of the data sets constituting saidsub-band to which dc luminance information of the signal is at leastpredominantly confined and of at least two of the data sets constitutingsub-bands to which dc chrominance information of the signal is at leastpredominantly confined are less than the average of the amounts ofquantization of the other data sets; and said predetermined ordercomprises, first, said one of the data sets constituting said sub-bandto which the dc luminance information of the signal is at leastpredominantly confined, second, said at least two of the data setsconstituting sub-bands to which the dc chrominance information of thesignal is at least predominantly confined and, thereafter, successivegroups of the other stored data sets in a sequence, as between saidgroups, of sub-bands containing ac luminance information of increasingspatial frequency.
 6. A method according to claim 1, wherein saidsub-bands make up a square array in the two-dimensional spatialfrequency domain.
 7. A method according to claim 6, wherein said squarearray is a 4×4 array.
 8. A method according to claim 6, wherein saidsquare array is an 8×8 square array.
 9. A method according to claim 1,wherein said digital video signal is separately spatially sub-bandfiltered in respect orthogonal spatial directions.
 10. A methodaccording to claim 9, wherein a field or frame of said digital videosignal is sub-band filtered in one of said orthogonal directions in afirst one-dimensional sub-band filter arrangement, stored, and thentransposed and sub-band filtered in the other of said orthogonaldirections in a second one-dimensional sub-band filter arrangement whichis of substantially the same construction as said first one-dimensionalsub-band filter arrangement.
 11. A method according to claim 9, wherein,in a first stage of the filtering in each of said orthogonal directions,said digital video signal is subjected to low pass filtering followed bydecimation by two and also to high pass filtering followed by decimationby two, thereby to produce two intermediate outputs, and, in at leastone subsequent stage of the filtering in each of said orthogonaldirections, each of said intermediate outputs produced in the previousstage is subjected to low pass filtering followed by decimation by twoand also to high pass filtering followed by decimation by two.
 12. Amethod according to claim 9, wherein said digital video signal issub-band filtered in a first one-dimensional sub-band filter arrangementconfigured to sub-band filter the signal in one of said orthogonaldirections, and then sub-band filtered in a second one-dimensionalsub-band filter arrangement configured to sub-band filter the signal inthe other of said orthogonal directions.
 13. A method according to claim1, wherein:said desired sequence in which the stored data is written tosaid quantizer is such that (i) al the data of said one of the storeddata sets constituting said sub-band to which the dc luminanceinformation is at least predominantly confined are quantized, (ii) afterwhich, upon each occurrence of an operation carried out for a number oftimes equal to the number of data in each stored data set, those datacorresponding to a respective one of the spatial positions in each ofthe remaining stored data sets are quantized in a predetermined order;said quantizing is controlled such that all the data written to saidquantizer in step (i) are quantized in the same amount, namely in theamount appropriate to said sub-band to which the dc luminanceinformation is at least predominantly confined, and each of the datawritten to said quantizer in each of the operations of step (ii) isquantized in an amount appropriate to the data set of which it forms apart; and said entropy encoding is controlled such that all the dataquantized upon being written to said quantizer in step (i) are subjectedto said first coding technique and all the data quantized upon beingwritten to said quantizer in all of the operations of step (ii) aresubjected to said second coding technique.
 14. A method according toclaim 13, wherein said predetermined order comprises successive groupsof the remaining stored data sets in a sequence, as between said groups,of sub-bands containing ac luminance information of increasing spatialfrequency.
 15. A method according to claim 14, wherein said groupscomprise legs of a zig-zag pattern connecting the data of the samespatial position in the different stored data sets.
 16. A methodaccording to claim 13, wherein:said digital video signal is a digitalcomposite color video signal, the stored data sets are quantized inaccordance with respective values which are such that the amounts ofquantization of each of said one of the data sets constituting saidsub-band to which dc luminance information of the signal is at leastpredominantly confined and of at least two of the data sets constitutingsub-bands to which dc chrominance information of the signal is at leastpredominantly confined are less than the average of the amounts ofquantization of the other data sets; and said predetermined ordercomprises, first, said at least two of the data sets constitutingsub-bands to which the dc chrominance information of the signal is atleast predominantly confined and, thereafter, successive groups of theother stored data sets in a sequence, as between said groups, ofsub-bands containing ac luminance information of increasing spatialfrequency.
 17. A method according to claim 16, wherein the amounts ofquantization of said sub-bands constituted by the data sets to which thedc luminance information and the dc chrominance information is at leastpredominantly confined are at least approximately the same as oneanother.
 18. A method according to claim 16, wherein the number of saidother data sets exceeds the number of the data sets constituting saidsub-bands to which the dc luminance information and the dc chrominanceinformation is at least predominantly confined.
 19. A method accordingto claim 16, wherein said composite color video signal is an NTSCsignal.
 20. A method according to claim 16, wherein said composite colorvideo signal is a PAL signal.
 21. Apparatus for compressing a videosignal, the apparatus comprising:a spatial two-dimensional sub-bandfiltering arrangement operative to filter a digital video signal to forma plurality of data sets constituting respective sub-bands of thetwo-dimensional spatial frequency domain; a store for storing said datasets for a field or frame of said video signal; a quantizer operative toquantize the stored data sets in accordance with respective values, saidvalues being such that the amount of quantization of one of the datasets constituting a sub-band to which dc luminance information of thesignal is at least predominantly confined is less than the average ofthe amounts of quantization of the remaining data sets; an entropyencoder operative to encode the quantized data sets, said entropyencoder comprising a first coding portion for coding quantized datarepresentative of dc luminance information and a second coding portionfor coding quantized data representative of ac luminance information;and sequencing means operative to control writing of the stored datafrom said store to said quantizer in a desired sequence, to control theoperation of said quantizer in accordance with said desired sequencesuch that each datum written thereto is appropriately quantized, and tocontrol the operation of said entropy encoder in accordance with saiddesired sequence such that each quantized datum is directed to theappropriate one of said first and second coding portions.
 22. Apparatusaccording to claim 21, wherein said entropy encoder comprises a switchfor directing quantized data to the appropriate one of said first andsecond coding portions and said sequencing means is operative to controlthe operation of said entropy encoder by supplying thereto a timingsignal for controlling said switch.
 23. Apparatus according to claim 21,wherein said sequencing means comprises a first sequencer operative tocontrol writing of the stored data from said store to said quantizer,and a second sequencer operative to control the operation of saidquantizer and to control the operation of said entropy encoder. 24.Apparatus according to claim 21, wherein said sub-bands make up, in thetwo-dimensional spatial frequency domain, a rectangular array having adimension of 8 in the direction of scanning of the video signal and adimension of 4 in the direction orthogonal thereto.
 25. Apparatusaccording to claim 21, wherein:said desired sequence in which saidsequencing means is operative to control writing of the stored data fromsaid store to said quantizer is such that, upon each occurrence of anoperation carried out for a number of times equal to the number of datain each stored data set, those data corresponding to a respective one ofthe spatial positions in each of the stored data sets are quantized in apredetermined order; said sequencing means is operative to control theoperation of said quantizer such that each of the data written to saidquantizer in each of said operations is quantized in an amountappropriate to the data set of which it forms a part; and saidsequencing means is operative to control the operation of said entropyencoder such that, for each of said operations, that one of thequantized data forming part of the data set constituting said sub-bandto which the dc luminance information is at least predominantly confinedis directed to said first coding portion and all of the other data aredirected to said second coding portion.
 26. Apparatus according to claim25, wherein said sequencing means is so operative that saidpredetermined order comprises successive groups of the stored data setsin a sequence, as between said groups, of sub-bands containing acluminance information of increasing spatial frequency.
 27. Apparatusaccording to claim 25, wherein:in order to enable compression of adigital composite color video signal, said quantizer is operative toquantize the stored data sets in accordance with respective values whichare such that the amounts of quantization of each of said one of thedata sets constituting said sub-band to which dc luminance informationof the signal is at least predominantly confined and of at least two ofthe data sets constituting sub-bands to which dc chrominance informationof the signal is at least predominantly confined are less than theaverage of the amounts of quantization of the other data sets; and saidsequencing means is so operative that said predetermined ordercomprises, first, said one of the data sets constituting said sub-bandto which the dc luminance information of the signal is at leastpredominantly confined, second, said at least two of the data setsconstituting sub-bands to which the dc chrominance information of thesignal is at least predominantly confined and, thereafter, successivegroups of the other stored data sets in a sequence, as between saidgroups, of sub-bands containing ac luminance information of increasingspatial frequency.
 28. Apparatus according to claim 21, wherein saidsub-bands make up a square array in the two-dimensional spatialfrequency domain.
 29. Apparatus according to claim 28, wherein saidsub-bands make up a 4×4 square array.
 30. Apparatus according to claim28, wherein said sub-bands make up an 8×8 square array.
 31. Apparatusaccording to claim 21, wherein said spatial two-dimensional sub-bandfiltering arrangement is operative to separately spatially sub-bandfilter said digital video signal in respective orthogonal spatialdirections.
 32. Apparatus according to claim 31, wherein said spatialsub-band filtering arrangement comprises first and secondone-dimensional sub-band filter arrangements of substantially the sameconstruction as one another, said first filter arrangement beingconnected to receive said digital video signal, storage means forstoring a field or frame of said digital video signal after it has beensub-band filtered in one of said orthogonal directions in said firstone-dimensional sub-band filter arrangement, and a transpose sequenceroperative to transpose the stored field or frame and write thetransposed field or frame to said second one-dimensional sub-band filterarrangement to be sub-band filtered in the other of said orthogonaldirections.
 33. Apparatus according to claim 31, wherein, for filteringin each of said orthogonal directions, said spatial sub-band filteringarrangement comprises a first stage having a low pass filter followed bya decimator for subjecting said digital video signal to low passfiltering followed by decimation by two, and also a high pass filterfollowed by a decimator for subjecting said digital video signal to highpass filtering followed by decimation by two, thereby to produce twointermediate outputs, and at least one subsequent stage which duplicatesthe construction of the previous stage whereby each of said intermediateoutputs produced in the previous stage is subjected to low passfiltering followed by decimation by two and also to high pass filteringfollowed by decimation by two.
 34. Apparatus according to claim 31,wherein said spatial sub-band filtering arrangement comprises a firstone-dimensional sub-band filter arrangement connected to receive saiddigital video signal and configured to sub-band filter said signal inone of said orthogonal directions, and a second one-dimensional sub-bandfilter arrangement connected to receive said signal filtered by saidfirst filter arrangement and configured to sub-band filter it in theother of said orthogonal directions.
 35. Apparatus according to claim21, wherein:said desired sequence in which said sequencing means isoperative to control writing of the stored data from said store to saidquantizer is such that (i) all the data of said one of the stored datasets constituting said sub-band to which the dc luminance information isat least predominantly confined are quantized, (ii) after which, uponeach occurrence of an operation carried out for a number of times equalto the number of data in each stored data set, those data correspondingto a respective one of the spatial positions in each of the remainingstored data sets are quantized in a predetermined order; said sequencingmeans is operative to control the operation of said quantizer such thatall the data written to said quantizer in step (i) are quantized in thesame amount, namely in the amount appropriate to said sub-band to whichthe dc luminance information is at least predominantly confined, andeach of the data written to said quantizer in each of the operations ofstep (ii) is quantized in an amount appropriate to the data set of whichit forms a part; and said sequencing means is operative to control theoperation of said entropy encoder such that all the data quantized uponbeing written to said quantizer in step (i) are directed to said firstcoding portion and all the data quantized upon being written to saidquantizer in all of the operations of step (ii) are directed to saidsecond coding portion.
 36. Apparatus according to claim 35, wherein saidsequencing means is so operative that said predetermined order comprisessuccessive groups of the remaining stored data sets in a sequence, asbetween said groups, of sub-bands containing ac luminance information ofincreasing spatial frequency.
 37. Apparatus according to claim 36,wherein said groups comprise legs of a zig-zag extending diagonally ofthe array pattern connecting the data of the same spatial position inthe different stored data sets.
 38. Apparatus according to claim 35,wherein:in order to enable compression of a digital composite colorvideo signal, said quantizer is operative to quantize the stored datasets in accordance with respective values which are such that theamounts of quantization of each of said one of the data setsconstituting said sub-band to which dc luminance information of thesignal is at least predominantly confined and of at least two of thedata sets constituting sub-bands to which dc chrominance information ofthe signal is at least predominantly confined are less than the averageof the amounts of quantization of the other data sets; and saidsequencing means is so operative that said predetermined ordercomprises, first, said at least two of the data sets constitutingsub-bands to which the dc chrominance information of the signal is atleast predominantly confined and, thereafter, successive groups of theother stored data sets in a sequence, as between said groups, ofsub-bands containing ac luminance information of increasing spatialfrequency.
 39. Apparatus according to claim 38, wherein said quantizeris so operative that the amounts of quantization of said sub-bandsconstituted by the data sets to which the dc luminance information andthe dc chrominance information is at least predominantly confined are atleast approximately the same as one another.
 40. Apparatus according toclaim 38, wherein the number of said other data sets exceeds the numberof data sets constituting said sub-bands to which the dc luminanceinformation and the dc chrominance information is at least predominantlyconfined.
 41. Apparatus according to claim 38 which is capable ofcompressing an NTSC composite color video signal.
 42. Apparatusaccording to claim 38, which is capable of compressing a PAL compositecolor video signal.